Support multiple GitLab hosts from environment variables

[rexim]

introduced in #154

[mfocko]

comma-separated string like GITLAB_PRIVATE_TOKEN="host:token,host:token" maybe?

[rexim]

@mfocko I don't really know. I don't even use the environment variables configuration. It was introduced in #139 by @jspaulsen

@jspaulsen is the format ok for you?

[jspaulsen]

@rexim I don't see any immediate issues with this formatting, so 👍