Fix 'start nova-bm-dnsmasq' hanging.

Dnsmasq was moved into foreground but 'task' was left in the job which
makes upstart block until the main process exits.

Also adding a 1 second post-start, respawn and proper respawn limit.

Fixes bug #1182120

Change-Id: Ifeb31cc269515e6476e2482ac856417b123be447