Otherwise pypi version is installed and later replaced which cause
reinstallation of cmmi parts when re-running buildout

As a result installing erp5.util[testnode] and slapos.cookbook before
slapos.core will install a first slapos.core that will need to be
replaced and cause reinstallation each time. ( I guess )

this is not possible with ENVIRONMENT=

This reverts commit e542746a.

This function factorize the code to instanciate a recipe in a fake
buildout.
The new feature is that if running in a buildout directory, the recipe
will reuse the eggs from this buildout instead of trying to install eggs
again.

some tests use commands such as cat or sleep, so use slapos provided
version.

we now use slapos.cookbook:wrapper instead

slapos.cookbook:egg_test was not relying on anything that could not be
achieved by a simple slapos.cookbook:wrapper.
slapos.cookbook:wrapper are easier to manage because we don't need to
synchronize changes to profile and changes to egg.

At same time, start using new --python_interpreter parameter to pass our
generated interpreter with eggs

this is needed because the test needs to execute `python setup.py
test` within a directory containing the tested egg.

The instance working copy is a git clone from the software working copy,
so it should be using the same revision.