Change from manual URL construction to using a lib

Makes the code cleaner to read and more resilient to changes

Fixes https://jira.coreos.com/browse/QUAY-940

oauth/services/gitlab.py

@@ -55,6 +55,6 @@ class GitLabOAuthService(OAuthService):
   def get_public_config(self):
     return {
       'CLIENT_ID': self.client_id(),
-      'AUTHORIZE_ENDPOINT': self.authorize_endpoint().to_url_prefix(),
+      'AUTHORIZE_ENDPOINT': self.authorize_endpoint().to_url(),
       'GITLAB_ENDPOINT': self._endpoint(),
     }