Updated openstack/openstack
Project: openstack-dev/devstack  58f1405665f72ae482aa34e09049a2847be1570e

Reference git.o.o DevStack repo rather than GitHub

In docs, use git.o.o URLs rather than GitHub URLs for the DevStack repo,
and don't mention GitHub when git.o.o is more appropriate.
Also, replaced GitHub logo with Git logo in quickstart.png.

Change-Id: Iab0006144f008963b8cb5be2d10ce0f360c0e6ca