proxy: adds oslo.cache, config support, & stevedore

This patch brings together oslo.cache, oslo.config, and stevedore to
provide pluggable, hierarchical catalogue caching for marconi proxy.

Here's the list of changes:
- add configuration/driver loading at the app level
- remove select from proxy storage driver - unnecessary intelligence
  at storage layer
- node.weighted_select -> partition.weighted_select (clearer name)
- forwarding logic further refactored, placed in own module
- caching logic placed in lookup module
- selector passed down at app level to handle round-robin state
  globally
    * open to becoming configurable
- adds several TODOs for a better proxy

Change-Id: I3bc568315e685486d63cdce3ec278c89e3f2b2bc
Implements: blueprint placement-service