Several places pin mock egg individually. There are pins to `mock = 3.0.5`
and pins to `mock = 2.0.0:whl`. Mock 3.0.5 is the last version that
supports Python2 and can be considered as "current" in our context today.

For nxdtest we are going to add pytest-mock, which depends on the mock,
which, if not consolidated, will need to add another `mock = 3.0.5`
somewhere.

-> Stop doing that and move mock pinning into stack/slapos.cfg.
This way it will be even easier for upcoming Python3 support, because in
stack/slapos.cfg we can add per-Python3 [versions] with pins adjusted as
needed for py3, but all still in one single place.

Leave `mock = 2.0.0:whl` pins intact to be updated when needed by
maintainers of corresponding software releases:

    software/slapos-sr-testing/software.cfg:mock = 2.0.0:whl
    software/slapos-testing/software.cfg:mock = 2.0.0:whl
    software/wendelin/software-kerastensorflow.cfg:mock = 2.0.0:whl