Add support for stateful jobs and execution controller pod priority in k8s

[godber]

As an operator, I need to be able to specify a k8s pod priority so that stateful workers and execution controllers are not pre-empted. See:

https://kubernetes.io/docs/concepts/scheduling-eviction/pod-priority-preemption/

[godber]

I think this means I need to let the operator supply the name of a priority class in the terafoundation config file.

[godber]

I am going to add the following Teraslice Master configuration setting:

    kubernetes_priority_class_name: {
        doc: 'Priority class that the Teraslice master, execution controller, and stateful workers should run with',
        default: undefined,
        format: 'optional_String'
    },

[godber]

I had forgotten that we never implemented a first class job property for stateful to indicate that workers shouldn't be disrupted without fully restarting the job. Internally we have just been using the label feature to indicate it like this:

    "labels": {
        "stateful": true
    },

I am not sure it's appropriate to depend on label contents for a feature like this, so I will probably just implement the preemption stuff for execution controllers first. Once that's working right, I might try and extend it to include the label thing and make notes in the code that this is a bad idea to do in the long run.