e2e tests gang scheduling

[gflarity]

What type of PR is this?

Tests for gang scheduling as per the 'Grove Multi-Node QA Test Plan'.

What this PR does / why we need it:

Implements e2e tests for gang scheduling test scheduling.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a API change?

NONE

Additional documentation e.g., enhancement proposals, usage docs, etc.:

[shayasoolin]

I reviewed the infra code (shared_cluster.go and setup.go) changes, and the first new test in gang_scheduling_test.go.
I stopped there since my main comment is that I strongly suggest to invest time on having as many utility test functions as possible, and then reuse them for the test cases themselves.
We'll not just gain reusability and make it easier to maintain, but the test cases themselves will get much clearer as they will show as high-level function calls that look ideally as the comment above the test case: a list of steps.
Since this requires a non-trivial refactoring, I prefer to review the test logic itself after it's done. Thanks!

[gflarity]

Thanks @shayasoolin. I abstracted as much as I could into utility functions. PTAL.

[gflarity]

@shayasoolin @sanjaychatterjee updated to incorporate last suggestions. PTAL.

[julienmancuso]

2 nits, otherwise lgtm. Great job