Use systemd-nspawn instead of docker for image

Using systemd-nspawn gives us more flexibility in what hardware the
agent can see. With this setup, the network is not virtualized, and the
/dev inside the container looks nearly identical to the one in the host.

Also added RestartSec=30s to the unit file because without this, systemd
will respawn the agent every 100ms if it dies (say, because DHCP hasn't
completed yet). Since it retries a max of 5 times per 60s, unless the restart value is
set higher it will be marked 'failed' forever and not retry. Adding this prevents that.

Change-Id: Ic64ab4b0824595683e5428464512594dd90eed83