systemd: specify container_ttys in environment

systemd: specify container_ttys in environment

The lxc.tty configuration item specifies a number of ttys to create.
Historically, for each of those, we create a /dev/pts/N entry and
symlink it to /dev/ttyN for older inits to use.  For systemd, we should
instead specify each tty name in a $container_ttys environment variable
passed to init.

See http://www.freedesktop.org/wiki/Software/systemd/ContainerInterface/ and
https://github.com/lxc/lxc/issues/419.

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