Automatic merge from submit-queue

compensate for raft/cache delay in namespace admission

When attempting to create an object in a just-created namespace, the admission plugin can reject the creation attempt for two reasons:

1. The cache of existing namespaces has not observed the creation watch event yet. There was already compensation to perform a live lookup if the namespace was not found in the cache.

2. The live lookup can be performed against a non-leader in a HA etcd setup, and can return NotFound if the non-leader etcd has not observed the creation.

This PR adds a delay, then rechecks the cache before proceeding with the live lookup. This gives both the cache and the backing storage more time to observe the create, before failing the request.