no need to specify 'python = ${buildout:python}' because it is the default behaviour.

add custom build for cryptography and cffi eggs that are required by pyOpenSSL 0.14.
pyOpenSSL is no longer binary egg.

pyOpenSSL requires cryptography and cffi, but cryptography tries to build cffi internally that may fail.

It was blocked by a missing find-links. Using slapos.cfg stack helps in defining all those details.

Won't fill /tmp anymore.

Decouple the seleniumrunner from the browser and the xserver.

Provides the xserver and the firefox hook.

seleniumrunner code (inside the recipe) still has to be merged with its
original code to prevent duplication.

This reverts commit a1717922.