corrected the support for selecting from multiple python interpreters

In the last pull request made by me a bug was introduced: the new feature require that the virtualenv module was installed in every python interpreter. This problem is easily detected reading the log of documentation build process for projects that use this options (like one i have). This pull request solve this by using an existing functionality of the virtualenv command that allow the definition of the python interpreter to use in a new virtual environment. This feature allow the use of the virtualenv installed in the machine as command no matter the interpreter it use.

I hope this is the final solution, best regards
Yeiniel