/etc/resolv.conf doesn't get populated if dns_domain is empty

[quanah]

There is no requirement for a dns domain to be populated, however if one is not defined in /usbkey/config, the smartos hypervisor fails to populate /etc/resolv.conf breaking name resolution within the hypervisor. i.e.:

....
dns_resolvers=8.8.8.8
dns_domain=""

results in a broken system. This appears to be due to a bug in /lib/svc/method/net-physical, where it checks for:

if [[ -n ${CONFIG_dns_domain} ]] && [[ -n ${CONFIG_dns_resolvers} ]]; then