Add missing package dependencies for horizon

We tend to always include horizon with other things that will install
these dev libraries, but it should be buildable by itself as well.

Change-Id: Icc8afaaa94a398277740059b43b0fdf989c07ce5
Fix formatting of yaml in heat/README.md

Markdown wants code blocks to be indented 4 spaces. Also properly format
the comments for yaml.

Change-Id: Icdbdadc663ccff2577f27e120448e1ecec358399