Automated procedure to hook requirement data migrations on system upgrade

While upgrading 3scale, in order to run System data migrations involving DML sequences, that are not implemented as Rails timestamped migration files (and neither they should!), we need to define an automated solution to hook those migrations into system's rollout process with no need of manual interaction of the user.

As sometimes such migrations are considered requirements of the new version, the solution should take that into account and prevent the application from being completely rolled out until the process finishes.

It should be analogous to what happens for DDL migrations, executed by system-app-pre-hook, although not necessarily using the same structures.

This data migration solution must work both for deployments based on Openshift templates or 3scale operator.