We have an extensive set of container config options to do this
for us, and doing this unconditionally breaks several use cases.
For instance, if we want to bind mount a /dev/shm using the
container configuration, then lxc-execute, then lxc-init will
rudely unmount the /dev/shm and remount it as a private tmpfs.
Signed-off-by: Serge Hallyn <shallyn@cisco.com>