allow to specify assignment driver

this is needed for read-only-ldap as per
http://adam.younglogic.com/2013/10/read-only-ldap-in-keystone/
which replaces hybrid auth
Merge pull request #160 from SUSE-Cloud/readonlyldapsupport

Add support for read-only-ldap
Add migration script for assignment driver

Also made this attribute required, to simplify things.