WIP: Deploy as a Knative Service

What does this MR do?

Teaches GitLab how to deploy an image as a Knative Service.

NOTE: An image must have a PORT environment variable defined for ingress and health checks. e.g. in Dockerfile:

ENV PORT 5000
EXPOSE 5000

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/58368#note_149868711

TODO

• Write a service object to a) create the namespace and b) deploy the Knative service and c) obtain the domain for the deployed service

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer