Added support for node affinity labels

[rosmo]

Adds support for node_affinity field in googlecompute source. Example:

  node_affinity { 
    key = "workload"
    operator = "IN"
    values = ["packer"]
  }
  node_affinity { 
    key = "workload"
    operator = "NOT_IN"
    values = ["notpacker"]
  }

I put WIP in the title because the test are giving me a panic which seems to be unrelated to my changes. However, I tested building images with and without node_affinity configs. Also I'm not sure what parts get autogenerated and I shouldn't be including in my PR.

The PR also touches the waitForState mechanism, because if a sole-tenant node is not available the operation state becomes DONE but there is a precondition failed error emitted, which is flagged as a retryable error and will keep the function spinning until timeout. I'm also fine with taking that out, in case you think it's too much.

Closes #93

[rosmo]

Removing WIP, since make testacc gives a full PASS. make test works too - figured out it really needs to be run on GCE (looks to be OS Login related).

[nywilken]

@rosmo thanks for the contribution. Change wise this is looking good to me.

I'm a bit curious about your comment " figured out it really needs to be run on GCE (looks to be OS Login related)." Are you implying there is an issue with how OS Login works. Or does node affinity only work when Packer is executed on GCE nodes?

I would like to see a bit more documentation around the node affinity config attribute. Specifically what are the requirements for using affinity labels and an example of what an actual config may look like.

[rosmo]

re: OS Login - I think the test suite was making an assumption about some username (testing@packer.io) where mine was different. Not a big deal, as it's completely unrelated to this.

I added a link to the Sole Tenancy (aka node affinity labels) to the documentation - the feature is mainly for a) compliance purposes, and/or b) building Windows images using BYOL (as per licensing terms BYOL licenses are not allowed to run on shared tenancy infrastructure).

[nywilken]

Thanks for making the updates. The changes look good to me.