host: Use libcontainer to start containers

[lmars]

We should support running containers using libcontainer for the following reasons:

• More control over assigned resources (e.g. capabilities, currently hard-coded by libvirt-lxc)
• Do away with most of containerinit (containers will be direct children of flynn-host so FDs can be shared in a more conventional way rather than passing them over Unix sockets, and libcontainer does most of the other setup like networking and setting rlimits)
• Close our longest open bug (well, hopefully 😄)
• Drop libvirt-lxc as a dependency (this would break in-place updates, so we would either need to keep libvirt-lxc around or not allow users to do an in-place update)
• Support running Flynn in Flynn (which would make testing things like cluster backup / restore much easier)

I have made a start on this on the host-libcontainer-runtime branch: https://github.com/flynn/flynn/compare/host-libcontainer-runtime