Improve setup time of tests in the docker image that runs soledad tests
In order to speedup the setup time of the tests in the docker image we need to:
- ensure that most of the python packages needed for running tests are cached in the image.
- ensure tox uses the cache for setting up the test environment.
- update the image often or whenever one of this packages release a new version.
(from redmine: created on 2016-08-13)