DevCentral tests

Summary:
We test if DevCentral is up and if every expected service
runs inside the container.

This detects the two main failure case:

• the IRC bot xessife is down
• the Phabricator daemons are down

It introduces a DockerContainer class to run a command inside
a container, after connecting to the relevant server with a
Docker client available by SSH.

Here, ssh dwellers.nasqueron.org docker exec devcentral ps auxw.

To run such privileged access tests, make test-full is introduced.

Test Plan: run tests in prod and some simulated failure mode

Reviewers: dereckson

Differential Revision: http://devcentral.nasqueron.org/D62