Allow configuring pod priority globally and per cluster.

Allow to specify pod priority class for all pods managed by the operator,
as well as for those belonging to individual clusters.

Controlled by the pod_priority_class_name operator configuration
parameter and the podPriorityClassName manifest option.

See https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/#priorityclass
for the explanation on how to define priority classes since Kubernetes 1.8.

Some import order changes are due to go fmt.
Removal of OrphanDependents deprecated field.