Add new worker specs in groups

Currently a subclass of SpecCluster can generate new workers for the spec with `SpecCluster.new_worker_spec`, which returns a name and a worker configuration. This is typically called by scale.

```python
while len(cluster.worker_spec) < target_workers:
name, spec = cluster.new_worker_spec()
cluster.worker_spec[name] = spec
```

However, in both HPC and GPU cases we want/need to create workers in groups. For this reason it might be nice to change the API here to return a dictionary of new workers

```python
while len(cluster.worker_spec) < target_workers:
d = cluster.new_worker_spec()
cluster.worker_spec.update(d)
```

See https://github.com/dask/dask-jobqueue/pull/307#issuecomment-525415832 for motivation. This also comes up with GPU work, where we want to add a new node and have one worker per GPU on that node.