Use pecan controllers for routing

Use pecan controllers for routing

Pecan defines several efficient mechanism for routing requests to
the appropriate controller, but the current code for Neutron's
Pecan WSGI server basically uses Pecan hooks to route requests.

This patch partially fixes that, removing the 'resource_identifier'
pecan hook and replacing it with explicit pecan routes between
controllers added at resource registration time.

All the remaining hooks, like attribute_population and
policy_enforments, which were relying on finding the resource
name in the pecan.request threadlocal variable have been updated.

This patch also:
- ensures the appropriate plugin is always selected for a given
  resource
- add a common NeutronPecanController base class for the classes
  CollectionsController and ItemaController
- Fixes the way in which plurals and singulars are handled in
  neutron.api.v2.resource_heper

Change-Id: I4ec0d2276c3974117b497228d289c3fb0dc5a140