Add the doc build tools

tools/build_docs.sh generates the devstack.org website from the static
pages and generated pages created by running shocco against a DevStack
checkout.

Change-Id: I61dc3d56e4a4832a9ddd1904dd8af65c15a17e50
Merge branch 'fix-get-packages' into dt-test
Merge branch 'extras-hooks' into dt-test
Merge branch 'fixup-stuff-rhel6' into dt-test
Merge branch 'docs-shocco' into dt-test