Add PullPolicy config option for kubernetes

What does this MR do?

Allows the user so specify a image pull policy for build-images used (like with the docker executor).

Why was this MR needed?

May speed up CI steps when using versioned build images (rather than :latest)

Are there points in the code the reviewer needs to double check?

I'm unsure about the code duplication in common/config.go. Maybe it would be better/more clean to map the docker syntax for pull_policy to kubernetes values?

Does this MR meet the acceptance criteria?

  • Documentation created/updated
  • Tests
    • Added for this feature/bug
    • All builds are passing
  • Branch has no merge conflicts with master (if you do - rebase it please)

What are the relevant issue numbers?