GitLab has a notion of "external URL" - the canonical "frontend" URL the
server is reachable through: this URL is used as prefix to show
e.g. git-clone URL for repositories, etc, even if a server can be
reachable via several frontends.

Add external_url handling to slapos instance.

NOTE whether to use https or not is also defined by external_url, in
particular by external_url scheme.

/cc @kazuhiko, @jerome