CLOUDSTACK-5061 - Cloudstack doesn't consider storage overprovisioning factor when using thin Provisioning over VMWare VMFS datastores

Fix is use the storage overprovisioning factor (global configuration parameter "storage.overprovisioning.factor") to calculate total provisioning capacity for storage space allocation over VMFS based storage pools as well.
There are two level of thin provisioning provided in VMware, storage level and file-level (VMDK) thin provisioning. in CloudStack, all volumes are provisioned with thin VMDK format, so at hypervisor level, we ALWAYS do thin provisioning. If storage vendor has the ability to provide storage level thin provisioning in addition to VMDK thin provisioning, it is also allowed since it is transparent to Cloudstack.

Signed-off-by: Sateesh Chodapuneedi <sateesh@apache.org>