OHAI-551

Add LXC notes, and full-content cgroup unit tests for LXC.

Change /proc/1/cgroup to /proc/self/cgroup, more consistent
with kernel documentation
[here](https://www.kernel.org/doc/Documentation/cgroups/).

Full-content examples of cgroups in OHAI-551 ticket
[here](https://tickets.opscode.com/browse/OHAI-551?focusedCommentId=47513&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-47513).