Service-bound HTTP requests

This introduces an abstraction for "service-bound request functions", which allow you to perform HTTP requests without knowledge of the tenant, name of the endpoint in the config file, region, or authentication.

It's based on the pure-http-refinements branch, so to see only the changes this branch makes, see here: https://github.com/rackerlabs/otter/compare/pure-http-refinements...service-bound-http
> CLOUD_SERVERS = ValueConstant('cloudServersOpenStack')

The "cloudServersOpenStack" string is specific to openstack deployment. Its value should be known to pick service in service catalog. The primary reason why I want to take service as arg is to avoid this string as it should be configurable and known only to auth internal implementation (code processing service catalog).