Install Guide: Brief ceilometer intro

Add a brief intro for ceilometer.

Change-Id: I629c8a95534e6052c3cecd5d048981eb36d7b434
backport: havana
(cherry picked from commit a83747456e68bbf116177e48752c59adb50a827c)
Merge "Install Guide: Brief ceilometer intro" into stable/havana