Migrate build_bins pypi downloading from FPM to pip (requests-aws)

This PR depends on PR #280 

Change the method for downloading requests-aws from fpm to pip download. This changes nothing in terms of what is downloaded/installed because requests-aws has no dependencies, and is pinned at version 1.5 both in fpm and in pip. 

The modification is necessary for PR #270 in order to install Sahara with pip. However, once the transition is made to pip, FPM won't work properly with a man-in-the-middle proxy - so we need to change the way requests-aws is downloaded and installed.

Pip download puts .tar.gz files from pypi in the pip-packages folder. In the installing recipe, it is enough to copy the necessary .tar.gzs, and use pip to install from the pip-packages directory.

Note - this will require a cookbooks/bcpc/files/default/cacert.pem file to avoid ssl errors with pip.
Some guard is needed here so that offline runs of build_bins.sh can run through if all downloads are already in place.