Add logging support to ovsclients_impl.py

[dceara]

Right now there's no easy way to inspect the commands that were executed by OvsClient instances in ovsclients_impl.py.

Adding support to log the commands would make it easier in debugging tests and/or replaying commands executed by the test without having to rerun the whole scenario.

One potential option would be to add command logging before calls to self.ssh.run(..).