gitlab.lister: Do not expect gitlab instances to have credentials

Only the main gitlab.com instance supports a rate limit (thus the
credentials need), others do not (as per summer 2018).

swh/lister/gitlab/lister.py

@@ -59,11 +59,9 @@ class GitLabLister(PageByPageHttpLister):
         params = {
             'headers': self.request_headers() or {}
         }
-        # Retrieve the credentials per instance
-        creds = self.config['credentials']
-        if creds:
-            creds_lister = creds[self.instance]
-            auth = random.choice(creds_lister) if creds else None
+        creds_lister = self.config['credentials'].get(self.instance)
+        if creds_lister:
+            auth = random.choice(creds_lister)
             if auth:
                 params['auth'] = (auth['username'], auth['password'])
         return params