Add mock for get_vapp_metadata

It's the same API method for vApps and VMs, so need to check both IDs.

This only returns a piece of custom metadata, not the system metadata that is also returned.
Add Mock for put metadata
Add metadata to mock vm
Add Mock for get_vms_in_lease

The only option implemented here is a filter by href as that is all we need for our tests. It can easily be extended as required.