Method to get vApp reimplemented according to vApp subclass.
list_vapps overridden method clears the cache in order to be able
to recognize newly deployed vapps.
Web server VM names all lowercase.
Disable also deleting of DB server Vm for testing purposing.
Method to retrieve VMs gets each VM through the Director method.
Error checking the href accessor method
Testing recomposition.
Launching recomposition and debugging the intermediate steps (creation of VMs for the Web servers).