apparmor: silence 'silent' mount denials

newer lxc uses 'silent' when remounting on shutdown.  Silence that denial too

Author: Jamie Strandboge <jamie@canonical.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>
doc: Drop lxc.pivotdir from Japanese lxc.container.conf(5)

Update for commit 2d489f9

Signed-off-by: KATOH Yasufumi <karma@jazz.email.ne.jp>
Acked-by: Stéphane Graber <stgraber@ubuntu.com>