Don't fall when just loading novav21 modules

this avoids loading warnings on salt-master that may not have
os-client-config installed, as salt tries to load them irrespective of
__virtual__() result.

Change-Id: I7a3a9c8390127563f6770c9b38b5b3a93d642364
Related-Issue: https://mirantis.jira.com/browse/PROD-26920
(cherry picked from commit 7fb58f8bb4d55e69a642863fbfee90169b044232)
diff --git a/_modules/novav21/common.py b/_modules/novav21/common.py
index 391eab4..87e3d75 100644
--- a/_modules/novav21/common.py
+++ b/_modules/novav21/common.py
@@ -14,7 +14,10 @@
 import logging
 import uuid
 
-import os_client_config
+try:
+    import os_client_config
+except ImportError:
+    os_client_config = None
 from salt import exceptions
 
 
@@ -24,6 +27,10 @@
 
 
 def get_raw_client(cloud_name):
+    if not os_client_config:
+        raise exceptions.SaltInvocationError(
+            "Cannot load os-client-config. Please check your environment "
+            "configuration.")
     config = os_client_config.OpenStackConfig()
     cloud = config.get_one_cloud(cloud_name)
     adapter = cloud.get_session_client(SERVICE_KEY)