feat: marconi-proxy forwarding

This patchset inttroduces forwarding. The following resources are now
passed on to marconi:
- claim-related requests
- message-related requests
- queue stats requests

Furthermore, on queue metadata operations, the queue catalogue is now
updated with the desired metadata.

Change-Id: I16fa20bc53523574b7e8c2e30556ed6ad7df5a76
Implements: blueprint placement-service
feat: define interfaces for proxy storage

This patchset introduces an interface for future proxy storage drivers
to work with. Along with the interface, the basic machinery for
writing unit tests for the storage drivers is exposed.

It introduces two controllers:
- PartitionsController
- CatalogueController

This should facilitate exposing stevedore entry points for defining
custom proxy storage drivers.

Appropriate exception classes are also exposed.

six is introduced as a dependency to take advantage of
six.add_metaclass.

Change-Id: I49f66a942dc24ef3e090132c6862529fa4ebfcec