Make resolver a public property

[stevewolter]

The custom resolver mattmoor@ mentioned seems to be a very workable solution for endpoint resolving (#88). I've made a PoC implementation in http://shortn/_0SyJs3BlK7 (Google-internal link) that uses a custom resolver to resolve endpoint versions after Docker digests were resolved. Could you PTAL?

[bazel-io]

Can one of the admins verify this patch?

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[stevewolter]

I signed it!

[mattmoor]

Generally this LGTM.

Travis will fail because this isn't from a branch on the main repo and so it doesn't hand out the secrets (bad design!).

Generally the process (which I will handle, but FYI) is to merge the PR into a branch of the main repo, then open a second PR to merge it into mainline (which will run integration testing properly).

A few minor comments below.

[mattmoor]

I would say that this script may need to invoke the default resolver, if the template may also contain image references.

[stevewolter]

Done.

[mattmoor]

Can you add this to the k8s_object signature documentation at the bottom of the README.md?

[stevewolter]

Done.

[stevewolter]

Thanks for the review! I addressed the minor comments. Let me know if there's anything else I need to be doing.