Use Cluster reference to reduce SDK calls

The vmware compute driver retrieves the cluster reference
for every spawn and volume operation. The solution is to
retrieve the reference once from the __init__() of
vmwareapi/driver.py and reuse the same in other methods.
This will reduce the number of VI SDK calls made.

Fix for bug 1130222

Change-Id: I2179733ee4ab1b2aea919de0ad4c10631cf005ea
Merge "Use Cluster reference to reduce SDK calls"