Block downgrade from icehouse to havana

Block downgrade from icehouse to havana

We are purging all downgrade methods from icehouse to havana because:
 1) havana is going to become unsupported during Kilo cycle.
 2) most people will upgrade from icehouse, while a minor percentage
    from havana
 3) downgrade use cases are mostly to revert after failed upgrades

See discussion in https://review.openstack.org/109952 for details.

This change removes all downgrade code in this migrations and raises
an exception in downgrade of icehouse_release.

Partially-implements: blueprint reorganize-migrations

Change-Id: I874dd2475da0404370dc7e53d7c43e0a8dce0d56