[common][CI] Configurable Priority Queue Load

- Use VCAP_TEST_PQUEUE_LOAD for the work load in unit test
  - Fall back to 100,000

  Test plan: pass unit tests

Change-Id: Ia14b5b01c0e6941714bcc0ca5175318e35f96fb6

common/spec/unit/priority_queue_spec.rb

@@ -38,7 +38,7 @@ describe VCAP::PriorityQueueFIFO do
 
   describe 'high volume' do
     it 'should be able to accept a bunch of random values and return them in non-increasing order' do
-      n  = 100_000
+      n  = (ENV['VCAP_TEST_PQUEUE_LOAD'] || 100_000).to_i
       n.times { val = rand ; @q.insert val,val }
       prev = @q.remove