Add IsLXCSupported()
Added tests

Added tests for the following cases:

* Mallformed cgroup file
* Missing cgroup file
* system juju is running on is not linux
use filetesting
Responded to feedback
Merge pull request #889 from gabriel-samfira/lxc-detection

Add IsLXCSupported()

This PR adds better LXC support detection. It will only return true if running on a bare metal machine or a fully virtualized machine running a Linux kernel.

On a LXC container and Windows it will return false.