Add support for self-managed GitLab instead of CodeCommit

[cnfait]

Describe the solution you'd like
CodePipeline now supports self-managed GitLab instances. Can we leverage that to optionally use GitLab instead of CodeCommit in SDLF?

• do not create SDLF repositories on CodeCommit during the devops account setup
• do not create team repositories on CodeCommit when deploying new teams
• it would be good to support both self-managed GitLab and GitLab.com.
• this is probably more work than it looks at first sight.

Additional context
In SDLF 1.x GitLab was supported through mirroring to CodeCommit. Here it is a bit different as the dependency on CodeCommit would be removed entirely.

[cnfait]

Assigning it to me but I'm not the one doing the actual work - it's just to avoid other people working on the same task for now.