VMware: use default values in get_info() when properties are missing

The properties that we retrieve from VirtualMachineConfigSummary are
optional ('summary.config.numCpu', 'summary.config.memorySizeMB') and
if they are missing we should return default values from get_info()

This patch also improves the unit test for get_info(). It removes the
test in test_driver_api.py (test_get_info() and test_spawn() are the
same) and adds a new test in test_vmops.py

Closes-Bug: 1317912
Change-Id: Ibbb28985d13de371fdac8a119ad820d25acc21be
Merge "VMware: use default values in get_info() when properties are missing"