gitlab.scripts.configure.secrets: handle duplicates, allow none

What does this MR do?

gitlab.scripts.configure.secrets: handle duplicates, allow none

- Remove duplicate entries between `required` and `optional`
- Allow disable of either with `none`

We handle both of the above via converting the inputs to Lists.
We remove duplicates by `range` over `required` to remove from
`optional`. We handle disabling of either via `none` through the use of
`without`. If `none` is the only entry, this effectively disables.

gitlab.scripts.configure.secrets: fix trim on calls

Fix / remove the trim on the calls to
`gitlab.scripts.configure.secrets` that resulted in the need for
`{{ end -}}`, as it was abnormal to our standard style.

Set GitLab Pages and GitLab Shell `optional` parameters to `none`
to match their current implementations.

Set GitLab Exporter `required` to `none` instead of ` ` to align.

Related issues

Closes #3443 (closed)

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