Don't create RethinkDB instance if it is already configured in this VM.

For example, from already having run ./rethinkdb.sh

I was seeing this go by while running `rethinkdb.sh` for each VM:

```
The path '/var/lib/rethinkdb/instances.d/default' already exists.  Delete it and try again.
rethinkdb: default: The instance has already started
```

This stops it from trying to copy the config if the config already exists, and also from trying to create the data directory for that config. Trying to start the service again seems like a safe thing to keep doing.