Provide kas on configurable host instead of ingress path

What does this MR do?

• Uses kas.<gitlab-host> by default.
• Drop option to have <gitlab-host>/-/kubernetes-agent path
• The host is also configurable.

Related issues

#2373 (closed)

https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11581

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened (See thread @tkuah @Alexand)

/cc @tkuah @ash2k @mattkasa @ggillies @mnielsen