Add node affinity and pod affinity/anti-affinity.

[shashwathi]

Fixes #116

Proposed Changes

• Allow Builds to specify node affinity, pod affinity/anti-affinity.

[google-prow-robot]

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[evankanderson]

/assign @imjasonh

[imjasonh]

I think we should try to add an e2e test for this, even if the affinity/anti-affinity is vacuously satisfied (e.g., affinity matching any node, anti-affinity matching no nodes)

That way we can at least check that specifying affinity doesn't break a build.

[shashwathi]

@imjasonh : Sure. I can do that

• I will close this PR and submit another one from new fork.