CI: Fix image_tag spec to account for gitlabVersion default

What does this MR do?

Tagging 6.9.0 pipeline failed on specs_without_cluster surrounding image_tag_spec.rb. This is down to the change in state behaviors, now that release-tools is setting the values for global.gitlabVersion. Previously, it would be unset at all times. Now, all stable branches will have a value. This broke an expectation for master as a default.

Changes here load in global.gitlabVerison from the values.yaml of the checkout, and use the same logic to prepend v when this value matches SemVer patterns.

value	tag
nil	:master
6-9-stable	:6-9-stable
15.9.0	:v15.9.0

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
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

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
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.