core: Supporting hard coded ldap servers per domain

Adding support for hard coded ldap server list per domain.
If such a list is configured for a given domain, no DNS SRV lookup
will be made to obtain the servers list.
This patch also removes ordering the ldap servers by score, and uses
the ordering algorithms instead.

Change-Id: Ic0914f2f6d9432ecac0cebee3790eae902a1329a
Bug-Url: https://bugzilla.redhat.com/894681
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>