Fixing validation exception for valid security group

Now casting the id of the allowed security group to a
six.text_type() to avoid type mismatching when using ids.

Change-Id: Iff2e442501d9c85c2b73c9a36114728830b04701
Closes-Bug:  #1384743
(cherry picked from commit fc1b5c8463248b8d142f46f310afae0645312b62)
Merge "Fixing validation exception for valid security group" into stable/juno