Make Juno migrations config independent

Make Juno migrations config independent

This patch amends migrations added after the icehouse release
and before the healing migration.
Migrations are changed in a way that they are not anymore
dependent on configuration parameters but are anyway aware of
the fact that the database has not yet been healed.

To this aim, amended migrations now will need to inspect the
current schema and cannot be anymore be used in offline mode;
this is consistent with the behaviour of the healing migration.

This patch does not remove the logic for generating and
managing configuration-dependent migrations. For this reason
upgrade and downgrade routines still accept the active_plugins
parameter, which will not be used.

Change-Id: I9d55a01c64ef555b7774099f497c9eea596aea6e
Partially-implements: blueprint reorganize-migrations