PAM: Use cache_req to perform initgroups lookups
    
    PAM responder has been already taking advantage of the cache_req
    interface, so this patch is just replacing some code that performs
    initgroups lookups by using cache_req to do so.
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/1126
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>