Provisioning and configuration being expensive time-wise, we create Google Cloud Platform images for each Docker/Kubernetes/Kubernetes CNI versions triple, and configure these once (instead of for each build), and then re-use these in subsequent builds, effectively implementing a 'cache' of images.

This way, Vagrant VMs and cloud-provided VMs are configured in the exact same way, hence avoiding configuration drift and subtle bugs associated to different versions of software.

This should help speed-up the build by provisioning test VMs while we run unit tests, and destroying them while we generate coverage reports.

There may be situations when Terraform/Ansible/etc. fail to provision test machines.
In these cases, we want to fail fast instead of waiting until we time out in circle-test-pre and circle-test-smoke.

Doing so in circle-dependencies-post arrives too late for background provisioning.

CircleCI keeps 10 nodes in order to speed up the build, but the default is 3, as this is the minimum required to run any test. This should make more sense as a default for a developer to quickly start an environment and iterate.

This should speed the build up, and provide faster feedback to developers.

…S=...".

This should make development easier by allowing developers to set HOSTS in their calling shell, and re-use the value for this env. var. when calling individual test scripts, e.g.:
```
cd test
eval $(./run-integration-tests.sh setup)
./200_dns_test.sh
```

This should be useful when we just want to type one make integration-tests / ./run-integration-tests command to make/start VMs/configure them/test, but also investigate potential failures afterwards.

…rent directory.

Sourcing failed when running make integration-tests.

We previously had 'use image' as a default, but not 'create image', which is inconsistent and leads to unexpected failures.