A new option '--host' for lxc-ps

Allow for an additional --host parameter to lxc-ps hiding all processes running
in containers.

Signed-off-by: Guido Jäkel <G.Jaekel@dnb.de>
Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>