refactoring around vapp_template and entity id

1) refactored template model to hold vcloud_attributes. Also instantiating
template only after it is loaded from fog_interface.

2) Added check on id returned by api for given entity. checking for its presence and
valid format.

3) changed the sequence of events in vapp.provision, we do not have to load
template or network config if vapp already exists.

4) using should_receive instead of stubs. stub calls can go unnoticed, also
they don't strcitly check for number of times a method was called and
the input argument