Adds instructions to upgrade Havana to Icehouse

Adds instructions to upgrade Havana to Icehouse in Ubuntu 12.04.4 LTS,
placed inside Operations/Upgrades, right below the Grizzly to Havana
upgrade instructions for RHEL.
These upgrade instructions intend to have similar formatting and steps
as the Grizzly->Havana ones, but taking into account the following:
- Be Havana->Icehouse specific;
- Considering Neutron as well;
- Considering the special and popular case of OVS to ML2 migration;
- Ubuntu 12.04 LTS only, _for now_.

Change-Id: I8d3cce0a2ceb306174c37d92382e201ff545be60
Closes-Bug: #1312660
Co-Authored-By: Matt Kassawara <mkassawara@gmail.com>