Amending the quickstart guide

*Updating to avoid use of manage.py directly.
*Making it more DevStack friendly.
*Adding a sample DevStack local.conf to allow for exercise Horizon
 against more services.
*Added simple developer instructions so new developers aren't tempted
 to work on the DevStack instance of Horizon.

Change-Id: I0840c63183e7cec3b08e4f2ce208479ff7552f60
Merge "Amending the quickstart guide"