test/venv: Fix the python version of the test environment

test/venv: Fix the python version of the test environment

Otherwise it will result in:
[ 59%] preparing venv for mgr-dashboard
New python executable in /home/jenkins/workspace/ceph-master/build/mgr-dashboard-virtualenv/bin/python2.7
Also creating executable in /home/jenkins/workspace/ceph-master/build/mgr-dashboard-virtualenv/bin/python
Installing setuptools, pip, wheel...done.
Running virtualenv with interpreter /usr/local/bin/python2.7

Even though the Cmake options contained:
  '-DWITH_PYTHON2=OFF' '-DWITH_PYTHON3=3' '-DMGR_PYTHON_VERSION=3'

Mostly due to the fact that
  src/tools/setup-virtualenv.sh
default to:
  PYTHON_BINARY="python2.7"

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>