lxc-start-ephemeral: Implement -n to match manpage

The -n/--name option of lxc-start-ephemeral was never implemented
even though it was documented in the manpage.

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