feat: split proxy API into admin and public apps

Rationale: when deploying the proxy, operators should be the only ones
that have access to the administrative functionality - primarily, the
management of partitions. This split makes it possible to deploy the
forwarding portion of the proxy independently of the administrative
portion.

The appropriate changes in tests, setup.cfg, and configuration files
(see: I7cf25e47ecff47934b50c21000b31308e1a4c8a9) were made.

Certain helpers that are reused for wsgi transport implementations
were extracted to a common location to aid reuse.

Unit tests were also simplified for the proxy, with the intent to make
them more thorough in coming patches.

New test requirement: httpretty
- used to perform request mocking during proxy transport unit tests

Change-Id: Ia26981a78c477a896370c48768e71f45c364c769
Implements: blueprint placement-service
Merge "feat: split proxy API into admin and public apps"