only verify SELinux setup on Red Hat variants
only verify SELinux setup on Red Hat variants

Current code uses '/etc/selinux' directory to determine
if check SELinux status. But ubuntu also have this directory.

scylla_setup only tries to disable SELinux for Red Hat variants,
this patch

Signed-off-by: Amos Kong <eb01d8f828a6c9a20be4534e72e049aaf41503df@scylladb.com>

scylladb/scylla-artifact-tests,scylladb/scylla-artifact-tests