Fix use of securitygroup/firewall_driver

As reported by Boon Lee, various parts of the documentation were referring
to 'security_group' instead of 'securitygroup'. Additionally, the section
was left out of one command to configure the firewall_driver.

http://docs.openstack.org/trunk/config-reference/content/networking-options-securitygroups.html

Change-Id: I148f6a96064bd6fd31f9f6abaf6985881c337aa9
Closes-Bug: 1302945
Merge "Fix use of securitygroup/firewall_driver"