Switch controller to actually call the plugins

Switch controller to actually call the plugins

This swaps out the controller methods to actually dispatch the
requests to the plugins and return the result. After this patch
merges, pecan should serve as a minimally working API server for
development purposes.

TODOs:
* AMQP server - this will not listen for AMQP messages
* Cleanup policy and context modules - they only serve a happy path
* Implement the notifiers
* Move the functional tests into an appropriate location
* Bulk operations
* Address any other of the TODOs in the code

Partially-Implements: blueprint wsgi-pecan-switch
Co-Authored-By: Brandon Logan <brandon.logan@rackspace.com>
Change-Id: If5dc76a2974d13d45f0cc88419bcccb3332576cf