Make configurable the creation of heat stack domain

Heat has graceful fallback code if stack_user_domain is not configured.

It is important that this be configurable, for testing and for those who choose to
not run with a stack_user_domain yet.

Change-Id: I08f017412636327853b3c606afd36ae93aa09288
Merge "Make configurable the creation of heat stack domain"
Merge branch 'master' of github.com:openstack-dev/devstack