Edit "OpenStack packages for RHEL and CentOS" considering which
releases are now supported:
- Add table for OS compatibility info
- Moved instructions relative to EOL OS or OS which only
feat unmaintained OpenStack releases
- Add TOC limited to depth 3 because Contributor Guide/RST
conventions/Titles limits heading levels
Change-Id: Ib3093ba9ebd2ff74db530b24c1b84a49a26bcaf8