AD_PROVIDER: ad_enabled_domains - only master
    
    We can skip looking up other domains if option ad_enabled_domains
    contains only master domain.
    
    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 49f38702e62bbd1728757063ba407444e6270952)