KubeCluster Input sanitising

In `dask_kubernetes.operator.kubecluster` we have `make_cluster_spec`, `make_scheduler_spec` and `make_worker_spec`. These are called by `dask_kubernetes.operator.KubeCluster` when creating a cluster or can be invoked directly and the output modified and passed to `KubeCluster`.

Today we aren't doing much in the way of input sanitisation in these functions, which means these functions can generate invalid manifests.

E.g in #665 it was raised that the cluster name can be set to a name that is invalid in Kubernetes like `foo_bar` which has underscores that are not allowed.

```console
$ python -c 'from dask_kubernetes.operator import make_cluster_spec; import yaml; print(yaml.dump(make_cluster_spec(name="foo_bar")))' | kubectl apply --dry-run="server" -f -
The DaskCluster "foo_bar" is invalid: metadata.name: Invalid value: "foo_bar": a lowercase RFC 1123 subdomain must consist of lower case alphanumeric characters, '-' or '.', and must start and end with an alphanumeric character (e.g. 'example.com', regex used for validation is '[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')
```

We should do more input validation on all the arguments that can be passed to those functions to ensure they will be valid input for the Kubernetes API.