Use static IP address for dhcpd

For containers with user-defined bridge networks, Docker creates an
"embedded" DNS server, that listens on the loopback interface.
`generate-dhcpd-conf` was trying to pass this local DNS server to the VM,
causing it to fail all DNS queries.

This change makes `generate-dhcpd-conf` return static IP addresses, by
default `8.8.8.8`,`8.8.4.4`. This seems like a reasonable enough middle
ground between "no DNS" and trying to proxy VM DNS requests to the
embedded docker DNS server.

This has the unfortunate side-effect that the VM cannot resolve the
names of containers on the same user-defined network (though it can
still connect to them by IP). Hopefully I'll eventually get around to
fixing this.