Support for requesting GPU resources on K8S

If GPU resources are required for executor PODS in a multi-tentan k8s cluster, the resource limits section needs to specify that GPUs are required [1]. This is a simple request:

resources:
  limits:
    nvidia.com/gpu: 1 # requesting 1 GPU

 A simple configuration limit could be specified (just like executor memory):

  --conf spark.kubernetes.executor.limit.gpus=1

[1] https://kubernetes.io/docs/tasks/manage-gpus/scheduling-gpus/

I have an implementation I will submit via a pull request.