lowercase rhel_image_gce to match GCP requirements (#436)

fatal: [localhost]: FAILED! => {"changed": false, "failed": true, "msg": "{u'domain': u'global', u'message': u\"Invalid value for field 'resource.name': 'CentOS-7-x86-64-GenericCloud-1704'. Must be a match of regex '(?:[a-z](?:[-a-z0-9]{0,61}[a-z0-9])?)'\", u'reason': u'invalid'}"}