To get all instances gcp_compute made a call to the Google API for each
zone separately. Because of this if all zones needed to be queried
fetching hosts lasted 30+ seconds. Now the module will use a single
query that will return all the instances, so the execution should last
just a few seconds.
This commit also suppresses a warning from the google-auth library about
using user credentials because if an Ansible user wants to use user
credentials, there is no need to warn him about it.
