Introduce organizations to the backend

[palourde]

As an operator, I want to be able to scope every Sensu objects to an organization so I can apply my business logic

• [x] The message bus has a knowledge of _organizations
  • topic prefix, e.g. "default:subscription:webserver"
• [x] The data store has a knowledge of organizations
  • [x] entities
  • [x] handlers
  • [x] checkconfigs
  • [x] events
  • [x] mutators
• [x] By default the organization used is default for all objects
• [x] Must specify org in API requests to create Checks/Handlers/Filters
• [x] Store interface must be made aware of Org
• [x] Entities must be scoped by Org
• [x] If no Org is specified when creating objects via API add Default org
• [x] Pipelined to pull filters, mutators, and handlers for the Event's Organization
• [x] Eventd is also aware of orgs
• [x] Keepalived must scope entities by org

[grepory]

For MVP of orgs, there's no reason to add org/env to the message bus queues. The objects traversing the queues are already tagged with org.

[grepory]

We're going to reject keepalives from agents until they start sending Organization information.