Update dev quick-start for devstack

The devstack section of the developer's quick-start documentation is
updated:
1. replaces the https://devstack.org link (doesn't work) with
   http://docs.openstack.org/developer/devstack/
2. clarifies how to configure usage of the PXE or agent driver

Change-Id: I2220f685c862832f73b28157355e11c7b49ce29b
Closes-Bug: #1400461