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.
This is a fallback. The global housekeeping used to crawl the tree to find new containers. Since we have inotify we don't need to do it now. We still do it just in case we missed any events (can happen if we get overloaded).