Test if Etherpad works correctly

Summary:
We test if:

• the container website is live
• our plugin for /metrics is installed
• Wolfplex API can read the list of pads

This uses phpunit as testing framework.

Fixes T512.

Test Plan: Run the tests in various Eherpad installation conditions

Reviewers: ledesillusionniste

Subscribers: Kaliiixx

Maniphest Tasks: T512

Differential Revision: http://devcentral.nasqueron.org/D28