Generate recent changes list from git log

The list of recent commits to the DevStack repo is now generated
by tools/build_docs.sh from the last 6 month of the git log.

Change-Id: Idefdb5d5188ab7e743af131e3d6cb132382d25d4
Merge "Generate recent changes list from git log"
Merge branch 'master' of https://github.com/openstack-dev/devstack