Fix adding heat_stack_user role via v3 API

A regression was introduced via 301a894 because it expects
keystoneclient functionality which is not yet merged/released,
so instead of passing the name to roles.list() we must filter
the results locally to lookup the role ID.

The lack of filtering support is a general issue with the v3
keystoneclient interfaces, so when it's fixed we can remove
this workaround.

Closes-Bug: #1271190
Change-Id: I1b29cc131496e4b19493a8cd8e6d4ed913db0938
Merge "Fix adding heat_stack_user role via v3 API"