Revert "Stop using Knapsack Pro"

This reverts PR https://github.com/consul/consul/pull/3812

bin/knapsack_pro_rspec

@@ -0,0 +1,9 @@
+#!/bin/bash
+if [ "$KNAPSACK_PRO_TEST_SUITE_TOKEN_RSPEC" = "" ]; then
+  KNAPSACK_PRO_ENDPOINT=https://api-disabled-for-fork.knapsackpro.com \
+    KNAPSACK_PRO_TEST_SUITE_TOKEN_RSPEC=disabled-for-fork \
+    bundle exec rake knapsack_pro:rspec # use Regular Mode here always
+else
+  # Queue Mode - dynamic tests allocation across CI nodes
+  bundle exec rake knapsack_pro:queue:rspec
+fi