Add version flag

[dkoshkin]

Is this a bug fix or adding new feature?
Fixes #21

What is this PR about? / Why do we need it?
This PR adds --version flag at the top level command.
Running aws-ebs-csi-driver --version will pretty print the JSON, with the same format as kubectl version.

This PR does not add support for printing the info in any other format.

What testing is done?
Locally built the image and tested:

➜  aws-ebs-csi-driver git:(version) ✗ docker run -it amazon/aws-ebs-csi-driver:0.1.0-alpha --version
{
  "driverVersion": "0.1.0-alpha",
  "gitCommit": "c7b3689df8a5cdf212ca80f9f0332ac3d518eefc",
  "buildDate": "2018-12-04T18:45:21Z",
  "goVersion": "go1.11.1",
  "compiler": "gc",
  "platform": "linux/amd64"
}

I am not aware of a way to unit test when using -ldflags, if anyone has some pointers would be glad to add a unit test.

[k8s-ci-robot]

Hi @dkoshkin. Thanks for your PR.

I'm waiting for a kubernetes-sigs or kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[coveralls]

Pull Request Test Coverage Report for Build 218

• 15 of 18 (83.33%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.6%) to 51.596%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pkg/driver/identity.go	0	1	0.0%
pkg/driver/version.go	15	17	88.24%

Totals
Change from base Build 217:	0.6%
Covered Lines:	582
Relevant Lines:	1128

---

💛 - Coveralls

[leakingtapan]

/ok-to-test

[leakingtapan]

@dkoshkin thanks for the PR. For the unit test, could you create a version_test.go and just test the struct and methods within the file to get us some coverage?

[leakingtapan]

/lgtm

[leakingtapan]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dkoshkin, leakingtapan

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