mark havana as EOL/no security support

[Tom Fifield]

"""When new OpenStack version is released it becomes "Current stable release, security-supported"" and previous version "Security-supported". "Current stable release" is the primary target for backports of bugfixes by the stable-maint team, other active branches may receive bugfix backports. "Security-supported" releases are target for the security fixes by the VMT OpenStack Havana stable release will reach EOL at Juno (N+2) milestone-3 and stable releases starting from Icehouse will reach EOL 15 months after their release (approximately N+3 milestone-2). """

This means that on release of Juno, Havana is no longer supported, and docs.openstack.org should be updated accordingly.