Code review comment for ~ltrager/maas:lp1876217

Adam updated me on the meeting with snapd that happened last night. My understanding is snapd will expose /etc/os-release in the future by default. As such I've updated this MP to try /etc/os-release, /usr/lib/os-release, and /etc/lsb-release. If an os-release file has the ID of ubuntu-core its skipped as that is the base Snap. The information is then gathered from /etc/lsb-release.