Improper pty permissions - missing mode=0620, gid=5

This fix is coming from Debian bug:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=720122

The reason for the hardcoded gid= and mode= is because of the fix for
CVE-2013-2207 which removes pt_chown from glibc and so requires proper
write access to devpts.

It looks like the "tty" group is guaranteed to be gid=5 on at least all
RedHat based and Debian based systems. So this hardcode gid shouldn't be
a big problem. If we however support any distro where that's not the
case, we'll need to implement an extra lxc.conf option and matching
template changes.

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