Add request to retrieve agents information by name

[mgmacias95]

Hello,

In this PR I implement the API call requested in issue #24: GET/agents/name/:agent_name. With this new API call, the user will be able to retrieve agent information by its name:

$ curl -u foo:bar -k http://127.0.0.1:55000/agents?pretty
{
   "error": 0,
   "data": {
      "totalItems": 2,
      "items": [
         {
            "status": "Active",
            "name": "node01",
            "ip": "127.0.0.1",
            "dateAdd": "2017-12-05 09:46:17",
            "version": "Wazuh v3.1.0-alpha1",
            "manager_host": "node01",
            "os": {
               "platform": "centos",
               "version": "7",
               "name": "CentOS Linux"
            },
            "id": "000"
         },
         {
            "status": "Active",
            "name": "centos",
            "ip": "192.168.56.105",
            "dateAdd": "2017-12-05 09:46:17",
            "version": "Wazuh v3.0.0-rc2",
            "manager_host": "node01",
            "os": {
               "platform": "centos",
               "version": "7",
               "name": "CentOS Linux"
            },
            "id": "001"
         }
      ]
   }
}
$ curl -u foo:bar -k http://127.0.0.1:55000/agents/name/centos?pretty
{
   "error": 0,
   "data": {
      "status": "Active",
      "group": "default",
      "name": "centos",
      "ip": "192.168.56.105",
      "dateAdd": "2017-12-05 09:46:17",
      "version": "Wazuh v3.0.0-rc2",
      "manager_host": "node01",
      "lastKeepAlive": "2017-12-05 09:42:43",
      "os": {
         "major": "7",
         "name": "CentOS Linux",
         "uname": "Linux manager 3.10.0-514.el7.x86_64 #1 SMP Tue Nov 22 16:42:41 UTC 2016 x86_64",
         "platform": "centos",
         "version": "7",
         "codename": "Core",
         "arch": "x86_64"
      },
      "id": "001"
   }
}
$ curl -u foo:bar -k http://127.0.0.1:55000/agents/name/node01?pretty
{
   "error": 0,
   "data": {
      "status": "Active",
      "name": "node01",
      "ip": "127.0.0.1",
      "dateAdd": "2017-12-05 09:46:17",
      "version": "Wazuh v3.1.0-alpha1",
      "manager_host": "node01",
      "lastKeepAlive": "9999-12-31 23:59:59",
      "os": {
         "major": "7",
         "name": "CentOS Linux",
         "uname": "Linux node01 3.10.0-514.el7.x86_64 #1 SMP Tue Nov 22 16:42:41 UTC 2016 x86_64",
         "platform": "centos",
         "version": "7",
         "codename": "Core",
         "arch": "x86_64"
      },
      "id": "000"
   }
}

It's also possible to select fields to be returned, just like the GET/agents/:agent_id request:

# curl -u foo:bar -k "http://127.0.0.1:55000/agents/name/node01?pretty&select=os_platform,id"
{
   "error": 0,
   "data": {
      "os": {
         "platform": "centos"
      },
      "id": "000"
   }
}
# curl -u foo:bar -k "http://127.0.0.1:55000/agents/name/centos?pretty&select=id,group"
{
   "error": 0,
   "data": {
      "group": "default",
      "id": "001"
   }
}

Best regards,
Marta