Add virtletctl version

[ivan4th]

• implement virtletctl version
• make sure it works with Virtlet release mechanism

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This change is 

[pigmej]

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Reviewed 38 of 38 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[jellonek]

Reviewed 9 of 38 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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cmd/virtlet/virtlet.go, line 180 at r1 (raw file):

	flag.Parse()
	if *displayVersion {
		printVersion()

Imo os.Exit should be called from main, not from above func, but I'm fine with that also...

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pkg/tools/version.go, line 91 at r1 (raw file):

			continue
		case exitCode != 0:
			errors = append(errors, fmt.Sprintf("node %q: error getting version from Virtlet pod %q: %v", nodeName, podName, err))

There should be probably shown exitCode as err is equal in this case to nil.
If it should have the same body it should be probably denoted as case err != nil, exitCode != 0:

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Comments from Reviewable

[ivan4th]

Review status: 36 of 38 files reviewed at latest revision, 2 unresolved discussions.

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cmd/virtlet/virtlet.go, line 180 at r1 (raw file):

Previously, jellonek (Piotr Skamruk) wrote…

Imo os.Exit should be called from main, not from above func, but I'm fine with that also...

Done.

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pkg/tools/version.go, line 91 at r1 (raw file):

Previously, jellonek (Piotr Skamruk) wrote…

There should be probably shown exitCode as err is equal in this case to nil.
If it should have the same body it should be probably denoted as case err != nil, exitCode != 0:

Done.

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Comments from Reviewable

[jellonek]

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Reviewed 1 of 2 files at r2.
Review status: 37 of 38 files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[pigmej]

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Reviewed 2 of 2 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable