Allow cert bundle to be specified via env var

[jamesls]

Requests has a feature where you can set an env var to point to a ca bundle: http://docs.python-requests.org/en/latest/user/advanced/#ssl-cert-verification, however due to a bug in requests, this code path is not used when creating requests yourself (as opposed to using Session.request). This means that we need to implement our own logic for retrieving this value.

We can either honor REQUESTS_CA_BUNDLE, or call it something more specific to botocore if we want.

[jamesls]

Fixed via #203