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
Merge "Reference git.o.o DevStack repo rather than GitHub"