Rework eggtest to use eggs installed by buildout
This should address two issues:
- Allow using a custom python interpreter where all eggs dependencies would already be installed.
- The test names did not have stable name, because they were the full path of the folder, which varies from one test node to another. This depends on changes in the profile done in slapos!309 (merged) .
This changes the internals of EggTestSuite so other tests reusing it ( I think we only have deploy-test ) will have to be updated once a new erp5.util is released. @luke I guess this won't be a problem and this would allow to fix that second issue for deploy test as well, so I did not consider being backward compatible here.