Allow StatefulSet PriorityClass to be configured

[ryanemerson]

It should be possible for a user to configure the PriorityClassName used by the cluster StatefulSet via the Infinispan CR:

kind: Infinispan
...
spec: 
  statefulset: 
    affinity: 
      ...
    priorityClassName: ""
    # Possible future configurations
    tolerations: 
      ...
    topologySpreadConstraints: 
      ...

Utilising a nested spec.statefulset structure provides increased flexibility that we can leverage for future enhancements such as #1801 and #1803

[Crumby]

@ryanemerson Wouldn't it be better to go with something like qos, scheduling or schedulingRules instead of statefulset (btw. it should be statefulSet for consistency). The latter sounds way to generic as basically anything could fit in there including bunch of stuff we already have defined elsewhere and in the same time it won't allow for full SS configuration (as it shouldn't).

[ryanemerson]

Very good points. I like scheduling, so it would be:

spec: 
  scheduling: 
    affinity: 
      ...
    priorityClassName: ""
    # Possible future configurations
    tolerations: 
      ...
    topologySpreadConstraints: 
      ...