ovstest: Add test-netlink-conntrack command.

ovstest: Add test-netlink-conntrack command.

Add a new test module to help debug Linux kernel conntrack development
unsing the netlink-conntrack module.

The tool has three uses:

* `ovstest test-netlink-conntrack dump [zone=zone]`

  shows a list of the connection table

* `ovstest test-netlink-conntrack monitor`

  displays the updates on the connection table, until killed with Ctrl-C

* `ovstest test-netlink-conntrack flush [zone=zone]`

  empties connection (and therefore expectations table).

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>