Device Registry: provision a device

[bennage]

Please see #201 for context.

We want an API that allows us to provision a device for authentication with the backend.

• POST a GUID respresenting the device identifier
• return JSON doc contain the endpoint and token (everything the device needs to connect to the event hub)
• return the appropriate response code if the device is already provisioned
• using the Service Bus APIs described in #156 create a token associate with the device id
• update the associated record in the registry with device status provisioned
• if there is no record in the registry, create a record for the device.