Ticket 47900 - Server fails to start if password admin is set
Bug Description: The server will crash at startup if the password admin is set.
Fix Description: When creating a password policy for the pblock, which we do
for add and modifiy operations, make sure the server is fully
up and running before searching backends for password admins.
Also, allow passwordAdminDN to be deleted from the cn=config.
https://fedorahosted.org/389/ticket/47900
Reviewed by: nhosoi(Thanks!)
(cherry picked from commit 4711de687174d6e5dd7b2c04782b62dc1cb90a6e)