AD: Add autofs provider
    
    https://fedorahosted.org/sssd/ticket/1632
    
    Adds the possibility to configure:
        autofs_provider = ad
    
    The AD autofs provider uses the rfc2307 (nis*) attribute maps. This is
    different (at the moment) from using autofs_provider=ldap with
    ldap_schema=ad.
    
    Reviewed-by: Ondrej Valousek <ondrejv2@fedoraproject.org>
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>
    (cherry picked from commit 03b859510dc13a13a456ca4aa94c0561a0e9684c)