Add default value '' for 'role' parameter in decort_group module.

library/decort_group.py

@@ -206,7 +206,7 @@ class decort_group(DecortController):
             bservice_id=dict(type='int', required=True),
             count=dict(type='int', required=True),
             timeoutStart=dict(type='int', required=False),
-            role=dict(type='str', required=False),
+            role=dict(type='str', required=False, default=''),
             cpu=dict(type='int', required=False),
             ram=dict(type='int', required=False),
             networks=dict(type='list', default=[], required=False),