Create conditional runtime environment for gcp-config docker image

[stephen-soltesz]

Currently, we've added runtime conditions to the stctl command for use during execution from Cloud Build. Instead, we want a runtime environment that interprets conditional directives like --execute_in_project=<foo> before calling exec on the remaining parameters passed to the docker image.

I imagine a new command cbenv or cbexec or cbopt or cbcond or similar, that accepts parameters from the environment and then execs the rest of argv if the conditions for execution are satisfied.

The first condition it should support is "EXECUTE_IN_PROJECTS=". With this directive it will be possible to have a single cloud build config with commands that should only be run in specific projects.

- name: gcp-config
  args: [ 'echo', 'hello world' ]
  env:
  - EXECUTE_IN_PROJECTS=mlab-sandbox,mlab-staging,mlab-oti

Additional ideas include conditional action on built-in environment variables -- https://cloud.google.com/cloud-build/docs/configuring-builds/substitute-variable-values For example, IF_TAG_SET, or IF_TAG_MATCHES=<pattern>.

[pboothe]

Done in the attached PRs