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.
`erp5.util.testsuite` has been extended to support a parameter to
specify which python to use.

This cause chicken and egg problem because 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. For now this is open issue. 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).

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.