Clean up stale LXD containers created by snapcraft

[kalikiana]

@sergiusens

A way to cleanup stale containers is needed, a cleanup strategy involving lxd’s last_used_at information is being worked on. How to expose this to the user is something we need to propose, it will most likely be a new argument to clean or a new command entirely.

Right now, SNAPCRAFT_CONTAINER_BUILDS=1 snapcraft clean will remove the container. Otherwise it will be kept around indefinitely.