watchdog: pass in default timeout on service start

This commit will allow other meta layers to easily override the default
timeout on watchdog start. Without passing the --min_interval option
there is no way currently for other layers to change this on startup.

Set the default of 30000 milliseconds to match the current default
defined within the phosphor-dbus-interface.

Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Change-Id: I6a25d5dd3da5704113c3f77d6f9737e0705b3d01