govc: Add an unreleased folder.place command for PlaceVMsXCluster

This change adds an unreleased govc command to exercise the
cross-cluster placement engine via folder.PlaceVmsXCluster.  This
command is limited to supporting the only a relocate placement type but
will be extended to support createAndPowerOn and reconfigure in the
future.  Bats tests are added to perform basic validation of the new
command.

This change also fixes the PlaceVMSXCluster action types to match the
types returned by the VIM API, specifically:
ClusterReconfigurePlacementAction => ClusterClusterReconfigurePlacementAction
ClusterRelocatePlacementAction => ClusterClusterRelocatePlacementAction

This change also improves the simulator to return the appropriate action
types based off of the placementType and improves the tests to validate
the action type.

Testing Done:
./govc/test/folder.bats
go test -v -count=1 ./simulator -run TestPlaceVmsXClusterCreateAndPowerOn
go test -v -count=1 ./simulator -run TestPlaceVmsXClusterReconfigure
go test -v -count=1 ./simulator -run TestPlaceVmsXClusterRelocate