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
Separate negative tests for test_quotas

Move negative tests from test_quotas.py to test_quotas_negative.py.
Some test cases in test_quotas.py which test the exceptions should
also be considered as negative test.
Move them to test_quotas_negative.py

Partially implements blueprint negative-test-files

Change-Id: Ib30e5b81d6c9519b6968aa8d6255fc10812dec3e
Merge "Separate negative tests for test_quotas"
Merge "Add unit tests for SSH client functionality"