vdsm_reg: fix ovirt_node without default bridge

This patch reflects the fact that nowadays ovirt-node usually
doesn't have a br* interface by default, thus the standing
assumption that it did lead to exceptions.

Change-Id: I01b761e4fa134850875a4118d40359df72ffd0ef
Bug-Url: https://bugzilla.redhat.com/988990
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/17730
Reviewed-by: Alon Bar-Lev <alonbl@redhat.com>
Reviewed-by: Mark Wu <wudxw@linux.vnet.ibm.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>