LB fetch request_func fixes

Click here to see changes against the dependent PR (#789): https://github.com/rackerlabs/otter/compare/pure-server-info-2...lb-fetch-request-func-fixes

This does a couple things at once, but they're very minor:

- First, request_func by default returns JSON-parsed objects. This branch updates it to pass json_request=False when a non-JSON result is desired.
- Second, request_func changed to require a ServiceType parameter to indicate which service to talk to. This branch passes ServiceType.CLOUD_LOAD_BALANCER.
- Third, retries for all HTTP requests are added with a policy of 5 max retries and exponential backoff starting at 2 seconds.