Cover agentconfig functionality by unit tests

[iliatsuprik]

This PR contains unit tests and minor changes in code for github.com/GoogleCloudPlatform/osconfig/agentconfig package.
Initial coverage: 58.1%, current coverage: 96.4%

Changes in the code of main app:

• move 5 seconds retry interval and runtime.GOOS into global variable. That's needed for mocking it in tests

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[google-oss-prow]

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This pull-request has been approved by: iliatsuprik
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[google-oss-prow]

Hi @iliatsuprik. Thanks for your PR.

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[burov]

/gcbrun

[burov]

/gcbrun

[burov]

/gcbrun

[burov]

/gcbrun

[burov]

I think we might benefit from using table driven testing more often in this CL, as well i see a lot of code duplication in test set-up, some of the code repetitions are low hanging fruits and can be just extracted to separate function as is.

[iliatsuprik]

@burov, thanks for the comprehensive review. I refactored most of the tests according to your comments. Could you check them again?

[burov]

/gcbrun