Introduce namespaces to cells about aurae

[dmah42]

yes we should isolate at the namespace level for each Cell.

• pid: this makes complete sense for a Cell to isolate. it means the first executable spawned will be "pid 1" in the namespace.
• net: i think this also makes sense for a Cell, but it might mean having some API to open up ports between Cells. it gives us some nice effects like assumption of eth0 in the Cell bridged to a virtual device in the Auraed space.
• mnt: unless we think Cells might want to share mount points, this makes complete sense to me.
• uts: i don't think we want this.. we want Cells to share the hosts hostname i believe. it makes it easy to reason about things from the Cell's POV.
• ipc: /shrug :)
• user: some security benefit to using this (can make user 0 unprivileged, and map UIDs to the host

[krisnova]

Started this work in #178

[krisnova]

We need to do some research on how cgroup namespaces and the other namespaces correlate to each other.

My PR #178 does the initial plumbing and new we need to start testing and validating our assumptions. I was not able to get the ps aux command to return the expected results for running in a new pid namespace.

We can (and should) look at a test harness of sorts for this!

[krisnova]

#183 also related

[krisnova]

I believe closed with #194