Instead of letting `python setup.py test` install the depencies, use
buildout way of installing the eggs.

This software use `interpreter` recipe of `zc.recipe.egg` to install a
python with all eggs pre-installed. This is a way to get all the
dependencies at install time instead of getting them at run time from
pypi when running `python setup.py test`.
`erp5.util.testsuite` has been extended to support a parameter to
specify which python interpreter to use.

One issue is that this way of installing eggs by buildout cause chicken
and egg problem: cloning repository containing `slapos.recipe.cmmi`
needs git, and to compiling git needs `slapos.recipe.cmmi`.
The consequence of this is that re-running software will install too
many parts again.
One solution for this would be to clone `slapos.recipe.cmmi` with a
`git` command provided by testnode or system package.
Another solution would be to not install `slapos.recipe.cmmi` develop
egg, simply install the egg from it's current pypi version while
installing the software (running tests will be from the git checkout
anyway).
For now this is open issue.

Another point of attention is that `python setup.py test` install the
requirements listed in `test_requires`, but `zc.recipe.egg` does not
provide a way of installing these. Some of our packages have `[test]`
entrypoints, in this case, the software installs the test entrypoints.
For others, we install the eggs.

Other improvements:
 * use a simple `slapos.recipe:wrapper` instead of `slapos.cookbook:egg_test`
 * fix the typo in repository name erp5-util-repository ->
   erp5.util-repository ( this mean we will have to fix the test suites in
   nexedi ERP5 )
 * document "what is this software" and a scenario of how this software
   can be used to develop slapos eggs.
 * switch to buildout-hash.cfg for easier template hash management.