[cmake] Add a DUNE_PYTHON_FORCE_PYTHON_VERSION variable

So far, a user could only distinguish python2 and 3. This makes it possible to also force a specific version given as a string.

Of course, the better solution would be if FindPythonInterp would accept a path hint just like every other find module does, but for some reason upstream is resistant to implementing this and I really don't want to ship our own find module for the python interpreter.

This fixes #118 (closed).