Install Guide: Clarify database node setup

Some people were not aware that they needed to install MySQL packages
on nodes. This patch rearranges the code so that we have separate
sections for controller and node and thus readers should easily
spot what needs to be done.

Change-Id: I104b949466b426e102d55bcb3c4a9cb72a0a6af9
Closes-Bug: #1250600
backport: havana
Document security group when using ML2 plugin

Since the ML2 plugin can concurrently support different L2 agents (or
other mechanisms) with different configurations, Neutron developpers
recommend setting firewall_driver flag in ml2 configuration.

Change-Id: I7f97128a955ded99400e25a5ef9a990260df3bf7
Closes-bug: #1262678
backport: havana
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Fix JSON example

Closes-Bug: #1240800

Change-Id: I1c61393b7e03bda9005e5111e35968f6bbb82cdd
author: diane fleming
Merge "Document security group when using ML2 plugin"
Merge "Install Guide: Clarify database node setup"
Merge "Fix JSON example"