Add GITLAB_ENVIRONMENT_* variables to db templates

This MR provides the GITLAB_ENVIRONMENT_NAME and GITLAB_ENVIRONMENT_URL env vars to the db-initialize and db-migrate jobs.

This is useful/essential when the db scripts must behave differently for each environment.

As an example, I require this functionality for projects where I am different configuration in development environments to that which is imported in production environments and for enabling/disabling environment-specific modules in CMS projects using the initialize/migrate scripts.