[!536] 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`).

See merge request !536

(cherry picked from commit 7e856817)

99f05bb7 Try to install Python virtualenv with pip before forcing get-pip onto users
92bf4520 Update documentation
1e9ae690 Create virtualenv during CI tests