Added gunicorn as server application manager

This patch adds gunicorn as dependency for starting and managing server
workers. Gunicorn allow users to chose whether to use workers based on a
sync process, eventlet or gevent. It also manages multiple workers, ssl,
process uid, process gid.

Implements blueprint transport-base

Change-Id: Ic328dbb937f8df69691911c6e6ff6b318bb1db34
Config module no longer remember cli args.

 - The .set_cli method is dropped;
 - .load method now accept an 'args' argument instead;
 - no cli args is read by default when 'args' is None.

Implements: blueprint config-module

Change-Id: Ic41904a584495170d8ca26c93c0d68e708469cc6
Queue stats returns claimed, expired, and total.

Change-Id: I665b385a0bf6ac2fd702615df94d440031c4928c
Implements: blueprint storage-base
Implements: blueprint storage-reference