Prior to this commit, this expected the api.token to be provided at task
initialization. That behavior has been kept for cli purposes. It's no good for
production purposes though (as this leaks the credentials in the scheduler db).
So now, the credentials is fetched from the lister's configuration file as the
other listers do.
Another change is the authentication mechanism which is slighly different. It's
not using a basic `auth` mechanism. It's expecting an `api.token` query
parameter so the `request_params` is overriden to provide that.
Related T1809
Annex:
gitlab.lister: Remove request_params method override (default one is enough now since b816212)
This should have been removed along with the code in b816212.
The request authentication has been reworked so that all listers use the same
credentials dict.
Related b816212
Related T1772