trestlebot init command should write provider CI files for GitLab

[gvauter]

Is your feature request related to a problem? Please describe.
The new trestlebot init command currently creates CI files Github. It should also write the equivalent CI files for Gitlab when a user selects that provider.

Describe the solution you'd like
When a user specifies --provider gitlab the init command should create the appropriate Gitlab pipelines files.

Describe alternatives you've considered
N/A

Additional context
See GitHub files as reference.

[gvauter]

Closing this based on decision implemented in #344 to no longer write provider files during init command.