Try to install a virtualenv with pip before annoying the user

When the virtualenv support was added to the build system, https://bugs.launchpad.net/debian/+source/python3.4/+bug/1290847# made us bail out completely on building a virtualenv with preinstalled pip. The situation has improved a lot since then; even Ubuntu 14.04 gets this right now as long as you install the right packages. So let's be nice to our users and try whether installing pip works before forcing them to do it manually or use get-pip. If a user insists on get-pip by setting DUNE_PYTHON_ALLOW_GET_PIP, we skip this additional step and keep using get-pip.

This shouldn't break anything, so I'll backport it to 2.6 as well, as it should make it much easier to use the virtualenv in CI images (e.g. for dune-pdelab-systemtesting, which requires dune-testtools).