Expose virtualenv --python as yaml option

[poros]

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).

[patricklucas]

+1

[ecanzonieri]

lgtm!