Add unit tests for SSH client functionality

Part of a push to add unit tests to Tempest for its internal
code. This patch adds unit tests for the SSH Client class
used by some of the functional integration tests for executing
commands inside server instances.

Note that there was an uncalled connect_until_closed method
that was removed from the tempest.common.ssh.Client class.

Closes-Bug: #1269948
Change-Id: I901900a27cc006d60f3ee6f5426ca6719a18f8ac
Addresses: blueprint unit-tests
3 files changed