Add general ceilometer concepts to guide

  The CLI guide did not contain the details of concepts such
  as meter, statistics and alarm. Since the next
  paragraphs included the commands to obtain these values, this
  would be confusing for newcomers.

  The fix includes the description of the concepts prior to the
  commands.

Change-Id: Ifdcc0e282e74838b9c420b140976c5767a06451b
Closes-Bug: #1263500
Move Networking scenarios section from Configuration Reference to Cloud Admin Guide

Moved networking-config-identity, networking-scenarios,
networking-adv-config, networking-multi-dhcp-agents from Config Ref to
Networking ch of Cloud Admin Guide

Change-Id: I1c14e9b7017fee976c2c80b6f0c8c7bcb15eb046
backport: none
Closes-Bug: #1255295
author: nermina miller
Merge "Move Networking scenarios section from Configuration Reference to Cloud Admin Guide"
Merge "  Add general ceilometer concepts to guide"