Make getting user-domain roles backend independant

There is nothing backend specific in geting the list of roles
for a user-domain, so we should move this function into backends
core.  This also has the affect of now ensuring that the kvs and ldap
support will work, provided the specific backend supports roles on
users and domains.  This is true today for kvs, but support in ldap
for domains is gated by other bugs.

Fixes bug #1131769

Change-Id: Id99accb33fd7cd8d6c37e64e140552c5bfe68349
Merge "Make getting user-domain roles backend independant"