nspawn: create empty /etc/resolv.conf if necessary

nspawn: create empty /etc/resolv.conf if necessary

nspawn will overmount resolv.conf if it exists. Since e.g.
default install with yum doesn't create /etc/resolv.conf,
a container created with yum will not have network. This
seems undesirable, and since we overmount the file anyway,
let's create it too.

Also, mounting a read-write /etc/resolv.conf in the container
is treated as a failure, since it makes it possible to
modify hosts /etc/resolv.conf from inside the container.