Introduce caps.isOvirtNode()

Change-Id: I6da5c94b5a8c11eace4813530570380c651e6d88
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/20809
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>