lxc-ubuntu: don't put devpts in $confdir/container/fstab

lxc-ubuntu: don't put devpts in $confdir/container/fstab

src/lxc/conf.c will explicitly mount it anyway.  Furthermore, the fstab
entry, which is getting processed first, did not specify -o newinstance.
This can cause the host's devpts entry mount options to change, as in
https://bugs.launchpad.net/ubuntu/+source/libvirt/+bug/607636.

Note - I messed up.  This was applied upstream, but I dropped it in
subsequent conversion to lxc-ubuntu template.  It therefore needs to
be reapplied.

Signed-off-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>