Install Guide: Attempt to fix heat install

Heat install does not currently work.

This patch takes steps towards fixing it.

Use connection instead of sql_connection also for Ubuntu.
Markup fixes

Backport: stable/havana
Closes-Bug: 1240396

Change-Id: I1d2ce9553667f6c3dace517c0f33219d35df9bc4
Merge "Install Guide: Attempt to fix heat install"