Install Guide: Make titles consistent

We have mixed styles in the guide with regards to capitalization
and do not use gerunds (_ING_ verbs). This fixes the remaining
pieces.

Change-Id: I20a45b77de267a946b8f5b24c4a54fb9461274d0
backport: havana
Install Guide: Merge two steps in Neutron Install

Steps 6 and 8 were referencing the same files while step 7
used another one. Let's merge the steps as proposed in the bug.

Change-Id: I8d379055db86f83bee181632aa84cec891cb9880
backport: havana
Closes-Bug: 1245914
Merge "Install Guide: Merge two steps in Neutron Install"
Merge "Install Guide: Make titles consistent"