O'Reilly edit: Logging and Monitoring

This commit addresses the technical comments from O'Reilly for the
logging and monitoring chapter.

A short introduction to Nagios was added as a sidebar and how to
calculate a user's quota usage was clarified.

Change-Id: I9e638d0dc3861a7a8cc73b1ef975c7ef0c7c2f0c
Merge "O'Reilly edit: Logging and Monitoring"