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

Change the method for downloading requests-aws from fpm to pip download. This makes no significant difference for requests-aws, but is better for packages that have lots of pip dependencies. For example, zabbix, graphite, and sahara can be downloaded from pip in this way.

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.