nova compute remounts to int max crashing host

[Boris Lukashev]

nova compute doesnt check whether /var/lib/docker/volumes/nova_compute/_data/mnt is mounted or does not unmount it properly on-exit resulting in 32768 mounts of the same path and a crash over time. It seems to remount recursively easily reaching that number in just a few minutes.

Happens on xena, yoga, and zed; seems to happen when ZFS is under docker (havent seen this on the AUFS or overlay machines but there's only one nova compute machine like that - rest are control plane).