Mount a tmpfs onto /root

cgmanager starts early enough that it may have a ro /.  Mount a
writeable tmpfs onto /root so that we can write lockfiles etc.
Without this, we sometimes end up hanging due to EROFS when doing
opening /root/.dbus-keyrings/org_freedesktop_general.lock.

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