Impala python should use a single virtualenv instead of separate scripts

There are 2 python environments accessible from the dev/test environment: the system python and a virtualenv python (in infra/python/). Python's virtualenv provides a nice way to set up a bash environment to use a specific virtual environment (virtualenv), but we have scripts that wrap python and py.test in the infra virtualenv: impala-python and impala-py.test. The latter seems to be the right python to use for almost all things, so it's not clear why we maintain these scripts rather than setting up the environment properly in impala-config.sh. We should see if we can just set up whatever is necessary in impala-config.sh (or something else that is run to set up a dev/test environment), and then rely on the right python being called.

See virtualenv docs and guide:
https://pypi.python.org/pypi/virtualenv
http://docs.python-guide.org/en/latest/dev/virtualenvs/