vdsm: move watchdog default params to WatchdogDevice

Watchdog device creation is currently handled in
buildConfDevices, creating unnecessary code pollution.
This patch moves the creation to WatchdogDevice class,
cleaning up buildConfDevices code. Side effect is that
we lose access to specParams if they are not passed from
engine.

Change-Id: Ic76f040bf78cbec3129569eb30fbf14447f742d6
Signed-off-by: Martin Polednik <mpoledni@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/19331
Reviewed-by: Dan Kenigsberg <danken@redhat.com>