phabricator.lister: Use credentials setup from configuration file

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