6.0 Update instructions

Most upgrade information is deleted but I left the header in place
to avoid build problems.

Note that no changes were made to the "How Fuel Upgrade Works" section
(http://docs.mirantis.com/openstack/fuel/master/reference-architecture.html#how-fuel-upgrade-works).

Change-Id: Ie5067c6d2b6e14cb144a9a9579a8ad91808dabbb