Use libffi and openssl in all environment variables.
Use custom python to install the eggs.
fixup! specify 'python = python2.7' in stack/slapos.cfg:[buildout] only.

no need to specify 'python = ${buildout:python}' because it is the default behaviour.
Merge remote-tracking branch 'origin/master' into erp5-component
we no longer need to specify python option in zc.recipe.egg.