heat_keystoneclient raise error if stack user role missing

Modify error path when the required role is not found, so we raise
an error and do not create the user if the required role is not
found.

Change-Id: Idbc8cc5821af8aca1442051a0a6c37e03b2522b9
Closes-Bug: #1277478
Merge "heat_keystoneclient raise error if stack user role missing"