Expose virtualenv --python as yaml option

Issue https://github.com/Yelp/pyleus/issues/54

The ``python_interpreter`` yaml option makes possible to build and run topologies with a different python interpreter than the default one.

I also substituted the ``requirements_filename`` if/else None that was above my new code with a get(), since the logic was the same.

Change is backward-compatible.

Tested running locally the ``examples/word_count`` topology with all python 2.6 and 2.7 combinations (default:2.6, option:2.6; default:2.6, option: 2.7; default:2.7, option: None; etc).