vdsm: change network sampling interval to 15 seconds

As discussed on engine-devel, there is no reason to poll libvirt more
often than engine polls vdsm. This patch will make polling in sync with
cpu (vm_sample_cpu_interval) polling.

Change-Id: I6c1763c1cc228791e8fa969f05d5bfcf80e2c5bf
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21257
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>