Add note regarding local port range

It is possible for the default port of a service (in particular,
keystone's administrative endpoint) to fall within a host's local
port range. This patch reminds users of this, and adds instructions
on how to:

- check a host's' port range
- check whether a port is already in use

Change-Id: If57d186153b4ffb8367d685a09675880b4fdaa04
Partial-Bug: #1261617
Merge "Add note regarding local port range"