upgrader: fix auto-generation of upgrades
- Fixes an issue with wrong upgrades being generated due to all SQLAlchemy models not being loaded.
- Adds option to create recipe command to overwrite the upgrade file if it already exists, as well as naming it from the CLI.
- Fixes an relative import bug in pre/post checks.
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>
NOTE: beware, patch amended by Tibor to use new file names