args: add name_optional flag

In preparation for adding support for -n to lxc-config.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
lxc-config: support getting container configurations as well

Also allow getting particular container configuration options via lxc-config.

$ sudo lxc-config -n u1 lxc.tty
4

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
wip