new Vdc class for accessing vdcs via Query API

* required for upcoming OrgVdcNetwork features
* immutable
remove tests against "language features" as per Sneha comment on vapp_spec
ensure we store the @id
improve get_by_name tests to check Query input
Merge branch 'master' into add_vdc_class
remove unnecessary multiple-vdc-with-same-name code & test
Merge pull request #83 from alphagov/add_vdc_class

new Vdc class for accessing vdcs via Query API