Integration test helpers

Pulled the helper code out from https://github.com/alphagov/vcloud-core/pull/48 as I need to use this for [#70499730] as well as [#70499718].

These two helpers allow us to share code that:

* Ensures the correct common environment / config is present for running our tests
* Provides a simple method to allow us to instantiate vAppTemplates as vApps/VMs to test against.