Document default exchange isn't specified in the config

Test Plan:
Post to /delivery/register_consumer the following payload:

{
	"anything": "anything"
}

(Flask considers request.json doesn't exist if the payload is empty,
this isn't a problem as we don't have any method accepting empty
payloads, as an API key will be required for register_consumer)

Get the following output to the console:

Client wants the default exchange, unspecified in config.
127.0.0.1 - - [06/Jul/2017 22:09:59] "POST /delivery/register_consumer HTTP/1.1" 400 -

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1000