PAM: Add domains= option to pam_sss
    
    Design document:
    https://fedorahosted.org/sssd/wiki/DesignDocs/RestrictDomainsInPAM
    
    Fixes:
    https://fedorahosted.org/sssd/ticket/1021
    
    Signed-off-by: Pavel Reichl <preichl@redhat.com>
    
    Reviewed-by: Sven-Thorsten Dietrich <sven@brocade.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>