wd: make watchdog device configurable

Add configuration option resources.watchdog_device allowing runtime
selection of watchdog device. Useful for newer servers having more
than one watchdog available (IPMI and iTCO).

Special value "off" disables watchdog in configuration rather than
just using build options. Useful when watchdog device is needed
elsewhere (SBD cluster stonith service).

Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>