Fix generation when engines define data migrations path

[gustavodiel]

Hey there!

There is a bug when having multiple paths defined by engines:

Let's say we have some engines at the engines folder
If any engine add a data migration path, when generating a new data migration with rails g data_migration Something then it'll join all paths into a monstrosity like db/data/User/MyUser/path/to/project/engines/engineA/db/data/User/....

This PR adds a new config option which is pretty much similar to the old data_migrations_path, except that it is only for setting the generation folder, so while multiple engines might add migrations path, only one migration gen path will exist

Edit: No further action is required aside from setting the data migrations path. The migrations gen path will have a default value which will behave exactly like before.

[ilyakatz]

Thanks for submitting the PR. Test seem to be failing, any ideas why?
Also, do you mind updating the docs so that other people know about this option.

[compwron]

This is probably affecting my team. I am commenting so I can keep track of this issue. We are cheering for you!