Sahara/api-v2

Refactoring the existing Savanna V1 api, making it easier for the user and separating the concerns of the user and deployer.

DEPLOYER = Person or entity who deploys and manages an Openstack installation and Savanna. Responsible for setup, configuration and management of Savanna.

USER = End Users who access the Savanna REST API via a http client, python-savannaclient or Savanna Horizon Dashboard.

High Level overview of Proposed changes

1. Refactor the format of the error response
2. Update the response codes and error response codes to reflect existing rest patterns.
3. Let's not expose the plugins directly via the rest API, instead expose services that are supported by the plugins.
   1. This allows a plugin to support more than one type of service.
   2. The user it not exposed to the backend.
4. Remove the images api.
   1. Savanna should be building a higher level service that abstracts the concepts on images from users.
   2. Expose services to users and manage plugins and images on the backend.
   3. Will help with the why not Heat argument.
5. Nomenclature change of node_groups to roles.