Enable v1beta3 API via --runtime_config=api/v1beta3 flag

This exposes the proper v1beta3 API endpoint when the user specifies
the --runtime_config=api/v1beta3 argument to the apiserver. v1beta3
is still considered experimental and subject to change.

--runtime_config is a map of string keys and values, that can be
specified by providing

    --runtime_config=a=b,b=c,d,e

Only the key must be specified, the value can be omitted.

Enables v1beta3 in hack/local-up-cluster.sh and hack/test-cmd.sh

hack/local-up-cluster.sh

@@ -92,6 +92,7 @@ sudo "${GO_OUT}/kube-apiserver" \
   -v=${LOG_LEVEL} \
   --address="${API_HOST}" \
   --port="${API_PORT}" \
+  --runtime_config=api/v1beta3 \
   --etcd_servers="http://127.0.0.1:4001" \
   --portal_net="10.0.0.0/24" \
   --cors_allowed_origins="${API_CORS_ALLOWED_ORIGINS}" >"${APISERVER_LOG}" 2>&1 &