govc: vm.clone ResourcePool is optional when -cluster is specified
govc: vm.clone ResourcePool is optional when -cluster is specified

When -cluster is specified, PlaceVm returns the HostSystem and Datastore
to use in the VirtualMachineRelocateSpec.  No need to set the spec Pool
field unless the -pool flag is also specified.

Fixes #1936

bruceadowns/govmomi,bruceadowns/govmomi,bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,vmware/govmomi