correct improper unmounting of shared volumes

Previously, as the child umounted the host tree after pivot_root, any
mount points under shared mounts on the host would be blown away. This
made volume mounts disappear after spinning up other containers, and
also would generally be impolite to the host system.

Signed-off-by: Nick Wade <nwade@pivotal.io>
stop installing strace now that it's in the image

Signed-off-by: Nick Wade <nwade@pivotal.io>
implement destroying volumes

lazy design: try umounting; if it fails for EINVAL, we're good; just
means the volume wasn't a mount point.

then, remove the volume recursively. for a host mount, this will just
remove the directory, as intended.
add comment for remounting as MS_SLAVE
mount changes should not propagate to host

remount the shared mounts as a slave to prevent this. otherwise, a
container can remount/umount its own shared volumes, which would
propagate the the host and cause weird behavior (e.g. destroying the
container could fail when unmounting its volumes).