Install guide: rework the ceilometer section

* misc editing details (spacing, formatting, capitalization, replaceable
  data)
* give configuration file examples where possible
* reduce the delta between distributions to make things more easily
  maintainable (and possibly clearer)

Change-Id: I2a8b3df1d26cb052c3ed71dc706f1243d1f0391f
backport: havana
(cherry picked from commit 98828402ccab087ed9923df61e6ccb987d0b9353)
Install Guide: document how to test the ceilometer installation

Change-Id: I4b5ea1f198f20083dc887d589e2d0b6a7f9116ba
backport: havana
(cherry picked from commit 2419e0c0d363e6a6a521594b7707c7575698868c)
Merge "Install guide: rework the ceilometer section" into stable/havana