Configurable probe termination grace periods (kubernetes#2241)

* Draft provisional KEP for probe grace period

* Address community feedback

* Update KEP reviewers/approvers

* Address reviewer feedback, adding feature flag

* Fill out the rest of the required sections for targeting alpha

* Update PRR requirements, respond to more feedback

* Add graduation criteria

Author: ehashman

keps/prod-readiness/sig-node/2238.yaml

@@ -0,0 +1,3 @@
+kep-number: 2238
+alpha:
+  approver: "@johnbelamaric"