remove miq_region_id column from database backups table

The region id should be encoded in the primary key, so we shouldn't need
a special column
Simplify backup naming by leveraging my_region_number and removing caching.

Update/clarify specs to reflect new behavior.
A region is the database so it's never destroyed.

DatabaseBackup rows are also not replicated so there's no issue with having them in a multiple region database.
We need to re-add the index for the foreign key id in the down.

To be symmetric, we should drop the index before we drop the column.
Merge pull request #1196 from tenderlove/miq_region

remove miq_region_id column from database backups table