MySql script not idempotent

Running `aws-database-migration-samples/mysql/sampledb/v1/install-rds.sql` failed due to [this bug]. Fixing it was simple enough, but re-running required a number of changes since the scripts are not idempotent. Ideally, there'd be a `DROP TABLE IF EXISTS table_name;` before each `CREATE TABLE`, and `DROP USER IF EXISTS` before the dms_user is created.