Add admin_user option to openstack::controller

This allows the creation of an admin user with a name other than
"admin", or disabling the creation of any admin user at all.

Use case: I have Keystone deployed with read-only LDAP, and I don't want puppet to try creating users, because it can't. Or, I want an admin user that isn't named "admin", because I want "jdoe" and "pfrost" to be admins, rather than have them share an "admin" password.