Name cloud provider(s) after the associated configuration key.

This is a first step towards having different cloud provider backends,
without loading all of them upfront; i.e., do not require Boto unless
we are actually using an EC2 cloud through the Boto API.

    modified:   conf.py
    renamed:    providers/cloud_providers.py -> providers/ec2_boto.py

This was part of the patch to support GCE, but may (and, IMHO, should)
be applied independently.