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
Merge "Install guide: rework the ceilometer section"