Runtimeclass scheduling updates

[tallclair]

Follow ups from the RuntimeClass scheduling API / architecture review. Concerns raised in the review:

• The term Topology is too ambiguous and overloaded
• @thockin was concerned about adding new special cased scheduler predicates.

This update addresses those concerns by:

• Renaming Topology to Scheduling
• Replacing the corev1.NodeSelector type with map[string]string. This makes the merging logic much simpler, so we can mix in the node selector in admission and leave the scheduler untouched. If we find a need for the more complicated node selector logic in the future, we can revisit this decision with an additional field.

/assign @thockin @dchen1107 @bsalamat @yastij
/sig node
/milestone v1.15
/priority important-soon

[yastij]

/lgtm

As we can still express constrains using nodeSelector and tolerations

[thockin]

Thanks!

/approve

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