c/r: refactor the way we pass data to criu/scripts

We previously wrote a bunch of files (eth*, veth*, and bridge*) as hard coded
files which we used as the names of interfaces to restore via criu's
--veth-pair. This meant that if people, e.g. gave a different bridge on their
new host, we would use our saved bridge in bridge* and try to restore to the
wrong bridge. Instead, we can just generate a new veth id (if the user hasn't
provided one), and use whatever the user configured values for the interface
name and bridge are.

This allows people to switch the bridge that they restore onto simply by
migrating the rootfs and config, and then changing the bridge name in the
container's configuration before running lxc-checkpoint.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>