Description of problem:

nova's database migrations are implemented using sqlalchemy-migrate, a defunct and unmaintained library. alembic is the anointed successor that is developed and supported by the sqlalchemy community. The Network (neutron) and Image (glance) services already migrated to alembic some time ago. It is now time for Compute (nova) to do the same. Doing so will reduce some tech debt in the project and bring usability improvements to operators, who can use the same tooling to run database migrations across multiple projects. This RFE tracks the work to migrate the migrations, providing a way for both existing deployments and new deployments to start using alembic for all migrations going forward.