Fix bug 2268: Authentication error when instantiating NS on GCP.

There was a bug in _reload_connection method of GCP connector which is called
when trying to reconnect after a previous error: the credentials parameter was
missing in googleapiclient.discovery.build call.

Change-Id: I38bcd1508cb3c45314f2fc76109c85c3279b9709
Signed-off-by: aguilard <e.dah.tid@telefonica.com>
(cherry picked from commit 6c7318f4ee720e17a8a6e95dc4ac0c551ee5da32)