Unify the way the default zone & region are set

cloudbridge/cloud/providers/gce/provider.py

@@ -180,9 +180,11 @@ class GCECloudProvider(BaseCloudProvider):
             with open(self.credentials_file) as creds_file:
                 self.credentials_dict = json.load(creds_file)
         self.default_zone = self._get_config_value(
-            'gce_default_zone', os.environ.get('GCE_DEFAULT_ZONE'))
+            'gce_default_zone',
+            os.environ.get('GCE_DEFAULT_ZONE') or 'us-central1-a')
         self.region_name = self._get_config_value(
-            'gce_region_name', 'us-central1')
+            'gce_region_name',
+            os.environ.get('GCE_DEFAULT_REGION') or 'us-central1')
 
         if self.credentials_dict and 'project_id' in self.credentials_dict:
             self.project_name = self.credentials_dict['project_id']

tox.ini

@@ -25,7 +25,7 @@ passenv =
     CB_USE_MOCK_PROVIDERS
     aws: CB_IMAGE_AWS CB_INSTANCE_TYPE_AWS CB_PLACEMENT_AWS AWS_ACCESS_KEY AWS_SECRET_KEY
     openstack:  CB_IMAGE_OS CB_INSTANCE_TYPE_OS CB_PLACEMENT_OS OS_AUTH_URL OS_PASSWORD OS_PROJECT_NAME OS_TENANT_NAME OS_USERNAME OS_REGION_NAME OS_USER_DOMAIN_NAME OS_PROJECT_DOMAIN_NAME NOVA_SERVICE_NAME
-    gce: CB_IMAGE_GCE CB_INSTANCE_TYPE_GCE CB_PLACEMENT_GCE GCE_DEFAULT_ZONE GCE_PROJECT_NAME GCE_SERVICE_CREDS_FILE GCE_SERVICE_CREDS_DICT
+    gce: CB_IMAGE_GCE CB_INSTANCE_TYPE_GCE CB_PLACEMENT_GCE GCE_DEFAULT_REGION GCE_DEFAULT_ZONE GCE_PROJECT_NAME GCE_SERVICE_CREDS_FILE GCE_SERVICE_CREDS_DICT
 deps =
     -rrequirements.txt
     coverage