Clean up queue if exchange doesn't exist

Summary:
When the API client calls /delivery/register_consumer, a queue is
first created, then bound to an exchange.

If the exchange doesn't exist, the queue has been created but is
unused. If we check the exchange exists through a passive exchange
declaration, we always introduce an extra call to the broker.

It's so probably faster to delete queue afterwards when this occurs.

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

{
	"exchange": "notexisting"
}

Get the following output to the console:

Deleting unused queue: delivery-020ea9cd-08c8-4029-83e2-28e21039c3cb
127.0.0.1 - - [06/Jul/2017 22:00:03] "POST /delivery/register_consumer HTTP/1.1" 400 -
Channel error: (404, "NOT_FOUND - no exchange 'notexisting' in vhost 'test'")

Queue wasn't present in the broker administration panel.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D999