Add tags to fleet server

[narph]

What is the problem this PR solves?

Issue elastic/elastic-agent#149

Add ability to set tags during installation and enrolment agent
so that different identical deployments of service can be differentiated in the dataset

The specific use case is described in: https://discuss.elastic.co/t/fleet-managed-elastic-agent-environment-deployment-name-custom-field/282565/6

sudo ./elastic-agent install -f --url=https://<feet manager>:443 --enrollment-token=<token> --tag=staging01 linux
or in case, the agent is already installed via package manager

elastic-agent enroll -f --url=https://<feet manager>:443 --enrollment-token=<token> --tag=staging01 linux

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Desired behavior:

 .\elastic-agent.exe enroll  --url=https://192.0.2.1:8220 --fleet-server-es=http://localhost:9200  --fleet-server-service-token=AAEAAWVsYXN0aWMvZmxlZXQtc2VydmVyL3Rva2VuLTE2NTA5NTg3Mjc0OTU6TmVHbEx4WWtUaVNETHNmNmNPV09lZw  --fleet-server-policy=fleet-server-policy --insecure --tag this-is-fleet,windows

{
        "_index": ".fleet-agents-7",
        "_id": "d1c66b32-72ed-4ab3-a36c-9660a5cb7125",
        "_score": null,
        "_source": {
          "access_api_key_id": "TW7SZIAB2-L_WJKcJz9m",
          "action_seq_no": [
            -1
          ],
          "active": true,
          "agent": {
            "id": "d1c66b32-72ed-4ab3-a36c-9660a5cb7125",
            "version": "8.3.0"
          },
          "enrolled_at": "2022-04-26T07:42:15Z",
          "local_metadata": {
            "elastic": {
              "agent": {
                "build.original": "8.3.0-SNAPSHOT (build: 2312423496161dd6f05148d2df0941d2c74570c8 at 2022-04-25 13:23:01 +0000 UTC)",
                "id": "d1c66b32-72ed-4ab3-a36c-9660a5cb7125",
                "log_level": "info",
                "snapshot": true,
                "upgradeable": true,
                "version": "8.3.0"
              }
            },
            "host": {
              "architecture": "x86_64",
              "hostname": "DESKTOP-K76UDQL",
              "id": "bc395ab9-1a1a-4312-ad0b-a05cceaf78f1",
              "ip": [
                "fe80::1d58:b17:4161:9414/64",
                "169.254.148.20/16",
                "2a02:a446:f25b:1:558d:72b2:74b6:7c67/64",
                "fe80::558d:72b2:74b6:7c67/64",
                "192.168.2.26/24",
                "fe80::d135:57b1:9b7f:e10a/64",
                "169.254.225.10/16",
                "fe80::6901:1ee0:60dd:c986/64",
                "169.254.201.134/16",
                "fe80::f09a:4481:bc48:e3e3/64",
                "192.168.21.110/24",
                "fe80::fc98:6ed8:6d25:3d98/64",
                "169.254.61.152/16",
                "::1/128",
                "127.0.0.1/8",
                "fe80::b54c:5acc:d25c:75e9/64",
                "172.23.192.1/20",
                "fe80::3186:c6c:cd37:92b8/64",
                "172.18.224.1/20"
              ],
              "mac": [
                "00:ff:6d:ef:82:d6",
                "cc:48:3a:b9:87:c0",
                "9e:b6:d0:3e:a8:c4",
                "ae:b6:d0:3e:a8:c4",
                "9c:b6:d0:3e:a8:c4",
                "28:c2:1f:52:ff:bf",
                "00:15:5d:a0:47:13",
                "00:15:5d:ff:9c:07"
              ],
              "name": "DESKTOP-K76UDQL"
            },
            "os": {
              "family": "windows",
              "full": "Windows 10 Pro(10.0)",
              "kernel": "10.0.22000.613 (WinBuild.160101.0800)",
              "name": "Windows 10 Pro",
              "platform": "windows",
              "version": "10.0"
            }
          },
          "policy_id": "fleet-server-policy",
          "tags": [
            "this-is-fleet",
            "windows"
          ],
          "type": "PERMANENT"
        },
        "sort": [
          1650958935000
        ]
      }

./elastic-agent.exe container -h

....
The following environment variables are provided as a convenience to prevent a large number of environment variable to
be used when the same credentials will be used across all the possible actions above.

  ELASTICSEARCH_HOST - elasticsearch host [http://elasticsearch:9200]
  ELASTICSEARCH_USERNAME - elasticsearch username [elastic]
  ELASTICSEARCH_PASSWORD - elasticsearch password [changeme]
  ELASTICSEARCH_CA - path to certificate authority to use with communicate with elasticsearch
  KIBANA_HOST - kibana host [http://kibana:5601]
  KIBANA_FLEET_USERNAME - kibana username to enable Fleet [$ELASTICSEARCH_USERNAME]
  KIBANA_FLEET_PASSWORD - kibana password to enable Fleet [$ELASTICSEARCH_PASSWORD]
  KIBANA_CA - path to certificate authority to use with communicate with Kibana [$ELASTICSEARCH_CA]
  ELASTIC_AGENT_TAGS - user provided tags for the agent
....

[mergify]

This pull request is now in conflicts. Could you fix it @narph? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b add-tags upstream/add-tags
git merge upstream/main
git push upstream add-tags

[mergify]

This pull request does not have a backport label. Could you fix it @narph? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v/d./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[elasticmachine]

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• Start Time: 2022-04-29T12:24:23.632+0000

• Duration: 10 min 22 sec

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Test	Results
Failed	0
Passed	274
Skipped	1
Total	275

🤖 GitHub comments

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[ph]

@narph I would really like to see a test for enrollment, Looking at the code it seems we don't have anything already in place, so it's a bit painful. Can you take a look?

[ph]

LGTM.