(POOLER-128) Remove references to VM mutex when destroying

This commit updates destroy_vm to remove references to its mutex tracking object when destroyed. Without this change a VM that is destroyed will leave its mutex tracking object forever causing the pool manager memory footprint to increase.

spec/unit/pool_manager_spec.rb

@@ -726,6 +726,12 @@ EOT
 
         subject._destroy_vm(vm,pool,provider)
       end
+
+      it 'should dereference the mutex' do
+        expect(subject).to receive(:dereference_mutex)
+
+        subject._destroy_vm(vm,pool,provider)
+      end
     end
 
     context 'when the VM destruction raises an eror' do
@@ -1690,6 +1696,26 @@ EOT
     end
   end
 
+  describe '#dereference_mutex' do
+    it 'should return nil when no mutex is dereferenced' do
+      expect(subject.dereference_mutex(vm)).to be_nil
+    end
+
+    it 'should return true when a mutex is dereferenced' do
+      subject.vm_mutex(vm)
+      expect(subject.dereference_mutex(vm)).to be true
+    end
+
+    it 'should dereference the mutex' do
+      mutex = subject.vm_mutex(vm)
+
+      subject.dereference_mutex(vm)
+
+      result = subject.vm_mutex(vm)
+      expect(result).to_not eq(mutex)
+    end
+  end
+
   describe 'sync_pool_template' do
     let(:old_template) { 'templates/old-template' }
     let(:new_template) { 'templates/new-template' }