Converge-service mock backend

In the converge-service you can use mocked data as the backend by passing in a config flag --elasticsearch or --mock

https://github.com/chef/converge-service/blob/master/cmd/converge-service/converge-service.go#L37

If mock is passed in the server starts the mock backend when starting up: 
https://github.com/chef/converge-service/blob/master/cmd/converge-service/converge-service.go#L82

This sets up which backend the backend package interface is using. 
Interface defined here: 
https://github.com/chef/converge-service/blob/master/backend/client.go#L18


The mock functions are easily implemented to return static data: 
https://github.com/chef/converge-service/blob/master/backend/mock/mock.go