Add google tag manager nonce config to omnibus

What does this MR do?

Related chart config MR: gitlab-org/charts/gitlab!2288 (merged)

Enabled to add GTM ID to our config. Adding this ID is part of a customer-critical-merge-request: gitlab!58494 (comment 740556436)

• Rollout issue: gitlab#346322 (closed)
• MR introducing GTM with the nonce version and the Feature Flag gitlab!58494 (merged) Related: gitlab-com/gl-infra/k8s-workloads/gitlab-com!1369 (diffs, comment 744923376)
• Staging config: gitlab-com/gl-infra/k8s-workloads/gitlab-com!1369 (comment 744923376)

How to Test

Please have a look at the local test plan for charts: gitlab-org/charts/gitlab!2288 (merged)

Related issues

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
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

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 the GitLab Chart opened (gitlab-org/charts/gitlab!2288 (merged))