cgproxy: remove proxydir if it exists

If /sys/fs/cgroup/cgmanager is not a mount that we can move, then
we need to rename it.  But if /sys/fs/cgroup/cgmanager.lower
already exists, then we have to remove that before we can
mv /sys/fs/cgroup/cgmanager to it.

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>