Use inotify to watch for new containers.

This reduces cAdvisor CPU usage below 1% in my CoreOS system.

We also reduce global housekeeping to every 60s as a fallback in case we
miss an event.
Yes :D today we only detect creation, deletion, and move but I can see us registering for modifications and exposing those. Today we get the spec on each request, but it may be nice to be able to find when it changes and show that to the user.