cleaning up vapp object

vapp should know only about its own fog_attributes. rest all can be
figured out from attributes.
refactoring vapp model, using request layer wherever possible

fixed constructor to initialize vapp model from fog attributes.
removed code to query vapp given vdc_name and vapp_name, instead using
fog query_engine since it uses service layer
divided fog_interface into service and model interface
smalll cleanup
making fogInterface session private

renamed attributes to vcloud_attributes to indicate that these are
attributes returned by vcloud(api)
resolving conflicts after fog_stub changes
Merge pull request #22 from alphagov/fixing_encapsulation_in_models

Refactoring launcher to improve code quality