Validate dask cluster name to contain only allowed characters

Having a "." in the cluster name leads to a hung cluster in the "Created" state:
```text
❯ kubectl get daskclusters
NAME WORKERS STATUS AGE
dmitry.balabka-cluster 1 Created 17h
```

While it is described in official k8s docs about Object Names, it would be great to have a validation in the dask library that raises at least a warning.
https://kubernetes.io/docs/concepts/overview/working-with-objects/names/

Code example:
```python
from dask_kubernetes.operator import KubeCluster

# Create a Dask cluster by calling Kubernetes API via kubectl command based on provided specificatoin
cluster = KubeCluster(
name="dmitry.balakba-cluster",
namespace="dask-operator",
)

client = cluster.get_client()

# Renders cluster configuration in the cell output
client
```