Basic passing Vdc::get_by_name integ test
Add missing get_vdc delegator, discovered by test

Splitting this into its own commit, to highlight that I'm
fixing a bug discovered whilst writing an integration test using Vdc#name

When running them initially, I got:

````
NoMethodError:
  undefined method `get_vdc' for #<Vcloud::Fog::ServiceInterface:0x007ff433291920>
````

This was due to a missing FogFacade delegator for get_vdc. This bug must have been
present for a while, indicating that we are not actually using Vdc.name or
Vdc.vcloud_attributes anywhere.
Test bad-data case for Vdc::get_by_name
Add Vdc::new constructor integ tests
Use "describe" for methods, not "context"
Merge pull request #65 from alphagov/70499754_vdc_integ_test

[#70499754] Vcloud::Core:Vdc integration tests & related bug fix