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?

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

## What are the relevant issue numbers?

See merge request !335