Webservice: address common.labels duplication

What does this MR do?

Webservice: address common.labels duplication

Address common.labels duplication within Webservice, as a result of the generator pattern & globalize templates.

Do this by:

• do common.labels merge in _datamodel.tpl definition so that we have consistency and inheritance
• remove gitlab.commonLabels
• change call pattern for webservice.commonLabels

TODO: tests in RSpec

Changelog: fixed

Related issues

Relates to #2582 (closed)

Closes #2728 (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

Closes #2728 (closed)