Updates VLAN documentation.

Brings the VLAN section up to date with essex, and documents how to
install the VLAN kernel module on the host.

Minor semantic tag adding and copy-editing in other sections.

Fixes bug 992846.
Fixes bug 752738.

Cherry picked from https://review.openstack.org/7337

Change-Id: I6b935d346b9c83e13667383282fab903733e7e1c