VMware image clone strategy settings and overrides

The VMwareAPI driver code currently spawns all virtual machines
copying the disks using a linked-clone strategy by default.
The VMwareAPI also offers a "full clone" strategy. This patch
makes the following changes and improvements.

* allow images to specify if they are "linked_clone" or not.

For technical discussion on the clone strategies and their trade-offs
see: https://www.vmware.com/support/ws5/doc/ws_clone_typeofclone.html

DocImpact: vmware_linked_clone metadata controls image-clone-strategy
Change-Id: I7a7ad57708202c8e8af5551b92a1e734185125f4
Blueprint: vmware-image-clone-strategy
Merge "VMware image clone strategy settings and overrides"