utilities: Fix conntrack flush command

utilities: Fix conntrack flush command

This patch fixes two problems.

1) The 'action "ovs-appctl dpctl/flush-conntrack"' does not do
anything because a command does not follow it.

2) Even after adding the command it still doesn't work - it must be
done before the stop_forwarding command.

In addition, make the message more human friendly.

Signed-off-by: Greg Rose <gvrose8192@gmail.com>
Acked-by: Aaron Conole <aconole@redhat.com>
Signed-off-by: Justin Pettit <jpettit@ovn.org>