Dev : Use a fixed version of py-spiffe version in Dockerfiles

[telliere]

pyspiffe library was updated earlier today, the update being not transparent API talking, it caused my code to be broken (SpiffeID object creation, workload_api usage ...) because my development environment hasn't been updated but the version of the package in the Dockerfiles isn't fixed.

We should fix this behaviour by selecting a version while running pip install of pyspiffe in the Dockerfiles

Although, the update is great and pyspiffe now supports fetching multiple SVIDs, thing that wasn't the case and that we had to workaround.

[telliere]

pyspiffe is under changes in order to be pushed to PyPi (see HewlettPackard/py-spiffe#133)
For this reason, a temporary solution is to fix the version used to a commit that we know worked (that has the setup.py ...)

[telliere]

pyspiffe now released under Pypi : https://pypi.org/project/spiffe/