Code review comment for ~samgilson/cloud-init:cloud-init-analyze-boot

One final round of review commments from me on this.
here's a pastebin of the changes if you think it'll work:

1. I think we need to document that we are currently working around an lxc issue in this code
with a comment to pointing to https://github.com/lxc/lxcfs/issues/292

2. we probably shouldn't subshell with subp to stat a file, but use python's os.stat

here's a pastebin of the changes I suggest:
https://paste.ubuntu.com/p/TYbddMbVJ2/