common/rc: improve checks for YP/NIS

At the 2018 spring NFS BAT, we have a number of systems that return
a valid string from `domainaname`, but don't have the ypcat binary.
If we don't have the yp-tools package installed, we're not going to
be able to use ypcat, so check for it and bail out.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>