cloud/elasticache: Ensure self.data is iterable

- self.data['SecurityGroups'] can return None causing traceback, if that
  is the case fallback to an empty list

library/cloud/elasticache

@@ -395,7 +395,8 @@ class ElastiCacheManager(object):
 
         # check vpc security groups
         vpc_security_groups = []
-        for sg in self.data['SecurityGroups']:
+        security_groups = self.data['SecurityGroups'] or []
+        for sg in security_groups:
             vpc_security_groups.append(sg['SecurityGroupId'])
             if set(vpc_security_groups) - set(self.security_group_ids):
                 return True