Update jenkins tasks to run integration tests

Adopt @dcarley method of populating vCloud credentials, as per
[#69718118], so we don't write them to disk on Jenkins server.
Ensure EdgeGateway integ tests have correct ENV

Our EdgeGateway integration tests require certain environment variables
to be set. These are documented in the README.md, but if this step is
missed then the tests fail unintuitively.

This commit adds the env checking code from other tests to ensure that
these are set before the tests are run.
Fix username,password,host Jenkins vars

In all our other vCloud integration tests, the following envvars
are used:

* API_USERNAME
* API_PASSWORD
* API_HOST

Fixing up this instance so that it matches this convention
Merge pull request #29 from alphagov/core_integ_tests_in_ci

[#70348090] Prepare vcloud-core integration test setup for CI.