add dir support

add dir support

It used to be supported with the lxc-create.in script, and
the manpage says it's supported...  So let's just support it.

Now

sudo lxc-create -t download --dir /opt/ab -n ab

works, creating the container rootfs under /opt/ab.  This
generally isn't something I'd recommend, however telling users
to use a different lxc-path isn't as friendly as I'd like,
because each lxcpath requires separate lxc-ls and lxc-autostart
runs.

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