Ubuntu 17.04 provides Docker 1.12.6, which has API version 1.24.
`dev_requirements.txt` specifies the docker-py 2.5.1, which by default
requests API version 1.30.
Hence when the SSH unit tests try to run the container specified in
`DockerizedSshDaemon` an error occurs
```
APIError: 400 Client Error: Bad Request ("client is newer than server
(client API version: 1.30, server API version: 1.24)")
```
On Ubuntu 17.10 something (probably Docker) appears to be accepting
connections, before sshd is fully ready. This results in a race
condition, and hence connection errors for the first few tests (2-3 on
my laptop).
testlib.wait_for_port() checks not only that the port can be connected
to, but also something resembling the sshd banner is sent.
Fixes#51
David Wilson