API v1.1 - Encapsulate message post bodies in a JSON object

This patch implements one of the breaking changes specified in
the API v1.1 spec, which requires clients to encapsulate the
list of messages to enqueue in a JSON object, when posting
those messages to the API.

Partially-Implements: blueprint api-v1.1-request-document-changes
Change-Id: I72ae87e1c479b6be6fbc8ce1fbaee89b2729e41a
Merge "API v1.1 - Encapsulate message post bodies in a JSON object"