dockerinit: set hostname

Set the hostname in dockerinit instead of with lxc utils.  libvirt-lxc
doesn't have a way to do this, so do it in a common place.
dockerinit: set IP address

Set the IP address in dockerinit instead of lxc utils, to prepare for
using libvirt-lxc.
Merge pull request #3201 from jpoimboe/libvirt-prereq-network

Set hostname and IP address from dockerinit