osmount: avoid stale propagated mounts

[petrutlucian94]

We'll need to prevent Coriolis mounts from being propagated to other namespaces belonging to Systemd services.

These would be leaked when running "unmount -R" in this namespace, being especially troublesome if the
minion gets reused. Stale mounts can prevent the FS from being remounted (e.g. if os-morphing gets retried).

[petrutlucian94]

There's also the option of limiting the scope to a given directory, although that would also have to be a mount, e.g.

sudo mkdir -p /mnt/coriolis
# bind /mnt/coriolis to itself so that we can modify propagation rules
sudo mount --bind /mnt/coriolis  /mnt/coriolis
sudo mount --make-private /mnt/coriolis

But then we'd have to ensure that all Coriolis mounts go under that path and also avoid rebinding it multiple times.

For simplicity we can stick with sudo mount --make-rprivate /.