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.