Fixed wrong assertion in test_vmops.py

There is wrong assert method in
nova.tests.virt.vmwareapi.vmwareapi.test_vmops.py:640
while checking a number of calls mocked method 'mkdir'.

We should use assertEqual(3, len(mock_mkdir.mock_calls))
instead of self.assertTrue(3, len(mock_mkdir.mock_calls))

But number of calling method 'mkdir' not always is 3.
If dict block_device_info contains key 'block_device_mapping'
with any information, method 'mkdir' wouldn't be called in method
self._vmops.spawn(). That is why corresponding check been added.

Change-Id: Ifc26e112b9c2974e0fb7a91ba39d5b2d887bd549
Closes-Bug: #1335859
Merge "Fixed wrong assertion in test_vmops.py"