[SSSD] [PATCH] AD: autorid compatibility should recommend the use of, default domain

[Stephen Gallagher]

Previously, we were failing to start if ldap_idmap_autorid_compat
was True but the default domain SID was unspecified. This is the
recommended configuration, but it is functional without it. There
is just a slight risk that the IDs will be inconsistent between
machines if the first user requested is not from the default
domain.

Fixes https://fedorahosted.org/sssd/ticket/1530