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Update priority admission to improve interoperability
Builds on https://github.com/kubernetes/kubernetes/pull/65722
Makes the following adjustments to the priority admission plugin:
* allows creation of pods to include an explicit priority field if it matches the computed priority (allows export/import cases to continue to work on the same cluster, between clusters that match priorityClass values, and between clusters where priority is unused and all pods get `priority:0`)
* preserves existing priority if a pod update does not include a priority value and the old pod did (allows POST, PUT, PUT, PUT workflows to continue to work, with the admission-set value on create being preserved by the admission plugin on update)
This should avoid the failures revealed by the kubectl tests exercising the pod API without any awareness of the priority feature
/sig scheduling
/cc @bsalamat
```release-note
kube-apiserver: the `Priority` admission plugin is now enabled by default when using `--enable-admission-plugins`. If using `--admission-control` to fully specify the set of admission plugins, the `Priority` admission plugin should be added if using the `PodPriority` feature, which is enabled by default in 1.11.
```
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