VMware: do not cache image when root_gb is 0

Save space on datastore when the instance size has root_gb of
0. There is no need to cache the disk as it already exists on
the datastore.

TrivialFix

Closes-bug: #1304593
(cherry picked from commit 9b14ae62a6396a2599c8a2ada1a50e3f69a2eddc)

Conflicts:
	nova/tests/virt/vmwareapi/test_driver_api.py
	nova/virt/vmwareapi/vmops.py

Change-Id: Iac75d97f40a1a91b8fa2df7fb4318fc2965fd93a
Merge "VMware: do not cache image when root_gb is 0" into stable/icehouse