GPO: Extract server hostname after connecting
    
    https://fedorahosted.org/sssd/ticket/2543
    
    The LDAP URI is not valid prior to connecting to LDAP. Moreover,
    reconnecting to a different server might invalidate the URI.
    
    Move reading the URI after the connection has been established.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    (cherry picked from commit ccff8e75940963a0f68f86efcddc37133318abfa)