AD: fall back to LDAP if GC is not available.
    
    AD provider went offline if the Global Catalog could not be connected although
    there was also the LDAP port available. With this patch, AD provider will
    fall back to the LDAP port before going offline.
    
    New boolean flag ignore_mark_offline was added to structure  sdap_id_conn_ctx
    If this flag is enabled function be_mark_offline will not be called.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2104