Plugin architecture

This spec suggests adding plugins support based on Stevedore [1].

Add hooks namespace 'ironic_discoverd.hooks' with calls:
* before_processing(node_info) -> () to run before taking any actions (even before validating the input). The intention is to affect node finding and data validation.
* before_update(node: Node, ports: [Port], node_info: dict) -> RESULT to run just before updating the node with the data.
  where RESULT is a tuple (node_patches, {MAC: port_patches}),
  any *_patches being a [JSON PATCH].

[1] http://stevedore.readthedocs.org/en/latest

NOTE: names were updated as part of https://blueprints.launchpad.net/ironic-discoverd/+spec/v1-api-reform