Migration Plugin

Currently many plugin specific items are specified in the mappings in constants.py.
It is very inconvenient for discoverability, not very scalable and requires plugins to follow multiple conventions.

To improve all above, a migrator class has been introduced.
- All plugin specific items should be moved there
- a migrator class for each plugin should be a Python entry point
- all plugin entry point should be loaded by the time of a migration plan validation or the migration itself
- plugin writer's section of README should be updated to reflect these changes.