Allow depth limiting git clones

Depending on how you are using devstack you probably don't need the
entire history of a project so we should allow people to specify a clone
depth to speed up the devstack process.

Change-Id: I804a5abcc80f6a81e915c0bb4dceae72486441a7
Blueprint: git-depth
Update docs section for new build

The devstack.org build process was recently changed to run as a CI post
job, publishing the site after every commit.

Change-Id: I05ddb353fae524178a25f28f2437d4fc635167f9
Merge "Update docs section for new build"
Merge "Allow depth limiting git clones"
Merge commit 'refs/changes/03/127603/4' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch
Merge commit 'refs/changes/11/129211/1' of git://gold.eng.hq.xensource.com/git/github/openstack-dev/devstack into tmpbranch