Use TaskConfigValidator for RunnerTask

[dirceu]

What are you trying to accomplish with this PR?

Fix part of #344 by using all supported TaskConfigValidator validations in kubernetes-deploy run (plus additional ones to validate args and template).

How is this accomplished?

In the first commit I enabled all validations from TaskConfigValidator and updated error messages in tests accordingly. I also chose to delete the unit tests we had for RunnerTask because:

1. There were only two and they were related to validation.
2. It would be too much effort to mock all kubectl and kubeclient calls for a low ROI.

In the second commit I created RunnerTaskConfig and RunnerTaskConfigValidator and moved the validation of args and template there to follow the same pattern for all validations.

What could go wrong?

The same as #533: because we need to unify validations that are done slightly differently we are changing the error class that gets raised as well as the wording. If people were rescuing these errors or matching text this will be a breaking change.

[dturn]

My two big concerns are if we really need a RunnerTaskConfig class. But also that we haven't lost coverage by the removed tests.

[dirceu]

also that we haven't lost coverage by the removed tests.

👍

I now added integration tests that make essentially the same assertions we had before, just in case.

[dturn]

Changes look good, one quick question.

[lei-lo]

LGTM 👍 Thanks for adding the logger statements :)

[dirceu]

Fixed rubocop warning and added changelog entry. Will merge once CI is green.