Remove unused @test_config from vapp_template_spec

This isn't used in the tests so remove it.
Rename catalog_item->vapp_template in config tests

These tests have no functional relation to Vcloud::Core::VappTemplate
because they only test config in/out. But they should be consistent with the
renaming about to occur in that class and vcloud-launcher.
Change arguments for VappTemplate#get

Swap the order of the template and catalog arguments so that they match the
`#get_ids_by_name_and_catalog` method, which can't easily be changed because
the order is reflected in the method name, and besides it reads more
naturally this way round - you want to get a specific item (first, can never
be a default) from a broader collection (second, could be a default catalog).

We temporarily decided not to so this because it means a matching version
change in vcloud-launcher. But we'll be releasing feature changes to schema
validation, so we might as well do this now.

Renaming `catalog_item_name` to `vapp_template_name` to match renaming about
to occur in vcloud-launcher. I'm not very attached to the `vapp_template_`
prefix because we're already in the VappTemplate class.
Remove Fog::ServiceInterface.get_[item_]catalog

These are no longer used by Vcloud::Core as of ba146c9. None of the other
tools use these methods either.
Remove Fog::ContentTypes::CATALOG[_ITEM]

These are no longer used by Vcloud::Core as of ba146c9. None of the other
tools use these consts either.
Merge pull request #49 from alphagov/70452106-rename_vapptemplate_catalog_item

[#70452106] Change arguments for VappTemplate#get