Fix bootstrapping zc.buildout 3+ from older zc.buildout

The big picture

Support running buildout bootstrap with zc.buildout 2.7.1+slapos20 on a buildout.cfg file containing:

[versions]
zc.buildout = 3.0.1+slapos001

in order to successfully install zc.buildout = 3.0.1+slapos001 and generate a bin/buildout that uses zc.buildout = 3.0.1+slapos001.

This will facilitate migrating to zc.buildout = 3.0.1+slapos001.

In detail

Remove hardcoded dependencies of zc.buildout in bootstrap: easy_install will anyway install all the actual dependencies of the pinned version of zc.buildout, and return a working set containing them all.

Before this, when the pinned zc.buildout had other dependencies than the ones hardcoded, they were installed correctly but bootstrap took only the hardcoded ones into account and wrongly failed due to the other dependencies being missing.

This is needed for buildout 3 because the dependencies now include pip and wheel in addition to setuptools.