Provide option for custom GitLab domain for GitLab Auth via environment variables.

[chriswachira]

I've found myself in a situation where I needed to configure GitLab authentication from a self-hosted GitLab instance. The global gitlab.com is baked into the GitLab Auth login handler code and it'd be good to have an option of specifying a custom GitLab domain via an environment variable to use for authentication.

For example:

export FLOWER_GITLAB_AUTH_DOMAIN="gitlab.mycompany.com"

The GitLabLoginHandler class will check for the custom domain from the environment and default to gitlab.com if none is present.

I am currently working on this. Let me know your suggestions.