network: mark container/VM/namespace networks as not required for online + disable...

network: mark container/VM/namespace networks as not required for online + disable DHCP lease persistency

These networks are not connections to upstream routers, but where we are
ourselves are the upstream router, hence it doesn't make too much sense
to require them to be up as default to determine if we are "online",
because they lead "in the wrong direction".

Also, disable DHCP lease persistency for these networks, since
container/VM/namespaces are generally shortlived, and typically have no
persistent identity. Moreover, the IP range we assign each VM/container
connection is just too small to permit persistency, as otherwise we'll
run out of leases way too quickly if VM/containers are restarted a bunch of
times with different MAC addresses (which I ran into).

I think these are better defaults, but of course these are only
defaults.