[backport:2.6][!544] Improve virtualenv placement
This backports the new options for virtualenv placement to the 2.6 release. While the change is not overly important in everyday use, it is really important for testing modules that use dune-testtools.
I changed the default for DUNE_PYTHON_EXTERNAL_VIRTUALENV_FOR_ABSOLUTE_BUILDDIR
to OFF
so that users don't
get an unexpected change in behavior on the release branch.