detect if inside Docker container

@btm the check for the files /.dockerenv or /.dockerinit look more reliable than what we did in the past.  It looks like /.dockerinit was added around July 2013, and has been around since v0.5.1.  It wasn't something that I knew about when I submitted the original pull request to add to Ohai.  As far as I can tell examining the code in this PR, this would provide better Docker detection that what we currently have, and I do not see that it breaks backwards compatibility.

Had to look back at my OHAI-551 notes. Prior to 0.9, Docker used the cgroup namespace /lxc/.  Post 0.9, it used the cgroup namespace /docker/.  The spec test  removed in this PR appears to be incorrect, and is certainly my fault.  There may have been good reason for it at the time, since we didn't yet have the virtualization.systems hash.

For docker guests, the presence of the `/.dockerinit || /.dockerenv` should detect most Docker instances excepting early first six month alpha builds (<= v0.5.0) of Docker.