Deployment spec patch

Adds deployment spec patching alongside pod spec patching. Allows to override deployment settings such as the deployment strategy.

This issue is related to many others (9 as of this point) and this feature will be a great boon for Operator configuration. Whether spec patching will remain in this form in the long term is unclear but I think we can safely add this field now and go from here in the future.

Docs are at gitlab-org/gitlab-runner!5766 (merged).

Closes #245 (closed)