Better OU= matching and on-the-fly user creation compatibility

parse_ou_from_dn had case sensitive OU= matching that was incompatible
with some LDAP servers. Adding the i flag makes the matching case
insensitive.

authenticate_with_sync_ou_to_group does not return the result from
authenticate_without_sync_ou_to_group. This breaks on-the-fly user creation. The user creation seems to work at least in the environment I'm dealing with with the added return statement.