diesel_cli doesn't fully respect MIGRATION_DIRECTORY

[matt4682]

When creating and running migrations through diesel migration generate and diesel migration run, diesel_cli looks at the MIGRATION_DIRECTORY environment variable and uses that as shown here.

However when you run diesel setup, it will only search for and create the default migrations directory.

Additionally running diesel database setup uses the run_pending_migrations function which also only looks at the default migrations directory, so migrations won't be run if they are configured and stored somewhere else.