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
diff --git a/HACKING.rst b/HACKING.rst
index d69bb49..3b86529 100644
--- a/HACKING.rst
+++ b/HACKING.rst
@@ -10,8 +10,8 @@
 Shell script was chosen because it best illustrates the steps used to
 set up and interact with OpenStack components.
 
-DevStack's official repository is located on GitHub at
-https://github.com/openstack-dev/devstack.git.  Besides the master branch that
+DevStack's official repository is located on git.openstack.org at
+https://git.openstack.org/openstack-dev/devstack.  Besides the master branch that
 tracks the OpenStack trunk branches a separate branch is maintained for all
 OpenStack releases starting with Diablo (stable/diablo).