CLOUDSTACK-4894: DestroyVirtualMachine API - added "expunge" admin-only parameter. When passed as true, the vm gets expunged immediately (false by default to preserve the initial command behavior)

Conflicts:
	api/src/org/apache/cloudstack/api/ApiConstants.java
	api/src/org/apache/cloudstack/api/command/user/vm/DestroyVMCmd.java