[!747] [python][cmake] fix handling of venv parameters

Merge branch 'fix-python-venv-debian-testing' into 'master'

ref:core/dune-common cmake parameters are passed as a list. The "-p
${PYTHON_EXECUTABLE}" option was passed as string, which lead to problems
setting up the venv on my Debian testing system.

We now pass all parameters as lists, just like it should be\...

1.  @core: perhaps someone from the cmake and/or python crowd should have a
    look\...
2.  I guess this wuld be god in 2.7?!

See merge request [!747]

  [!747]: gitlab.dune-project.org/core/dune-common/merge_requests/747