always quote dhcp-domain, otherwise dnsmasq can fail to start

the dnsmasq arg --dhcp-domain can't be passed to dnsmasq as empty
(i.e. --dhcp-domain= ) unlike many other arguments. If you do pass
it as empty, dnsmasq won't start, throwing a ProcessException which
then crashes the rest of nova-network.

Fix this by quoting dhcp-domain so that empty domains are allowed.

Fixed bug #1161027

This was found by grenade upgrade testing

Change-Id: Id24814339d8c8a709f5041f0aa98b10ba1e0e595
(cherry picked from commit 8d68ed3583df6ced4b4c8b6d6bb3569e03992cfe)