AD_PROVIDER: Add ad_enabled_domains option
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/2828
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit d6342c92c226becbdd254f90a0005b8c00c300dc)