Handle the default domain in LDAP

In order to maintain support for v2 clients, a default domain is always created in the sql backend and it is used for nothing else. The LDAP backend fails when there is no default domain found, though it is not required by the LDAP. We don't want to create a domain in LDAP unnecessarily because of read-only restriction in many LDAP user environment. We need to feed a fake domain to overcome this problem.
Creating this blueprint after discussion with Adam Young and Brad Topol.

Update (4/26/2013):
We do not have fake domain any more. We return our default domain unless a a particular default domain is created by the user.