AD: Provide common connection list construction functions
    
    https://fedorahosted.org/sssd/ticket/2810
    
    Provides a new AD common function ad_ldap_conn_list() that creates a
    list of AD connection to use along with properties to avoid mistakes
    when manually constructing these lists.
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>