fix drop *rp_scratch dbs

### Problem
Development database was being dropped when re-creating the schema.

### Solution
- Stop using drop rake task
- Drop scratch dbs using activerecord
- Make sure only the [default + shards] dbs are on the scratch config

I tested this local and it worked like a charm =)

review @dylanahsmith  or @hornairs  or @camilo 
cc @odorcicd