Test the cluster version matches configuration

We introduce a test to verify the running Kubernetes version against our
list of versions from the buildchain.

Note: we create a symlink, under `tests/versions.py`, pointing to
`buildchain/buildchain/versions.py`, for ease of importation within
tests.

Issue: #640

tests/conftest.py

@@ -84,6 +84,10 @@ def k8s_client(request, k8s_apiclient):
     def test_something(k8s_client):
         assert k8s_client.list_namespaced_deployment(namespace="default")
     ```
+
+    FIXME: this is not working as of right now, since `pytest-bdd` manipulates
+    fixtures in its own way through the various scenario/when/then/given
+    decorators.
     """
     api_name = getattr(request, "param", "CoreV1Api")
     api_cls = getattr(kubernetes.client, api_name, None)

tests/post/features/versions.feature

@@ -0,0 +1,5 @@
+@post @ci @local
+Feature: Check versions in the running cluster
+    Scenario: Check the cluster's Kubernetes version
+        Given the Kubernetes API is available
+        Then the Kubernetes version deployed is the same as the configured one

tests/post/steps/test_versions.py

@@ -0,0 +1,26 @@
+from kubernetes.client import VersionApi
+from pytest_bdd import scenario, then
+
+from tests import versions
+
+
+# Scenarios
+@scenario('../features/versions.feature',
+          "Check the cluster's Kubernetes version")
+def test_cluster_version(host):
+    pass
+
+
+# Then
+@then('the Kubernetes version deployed is the same as the configured one')
+def check_kubernetes_version(k8s_apiclient):
+    # NOTE: the `vX.Y.Z` format is used by Kubernetes, not our buildchain
+    configured_version = 'v{}'.format(versions.K8S_VERSION)
+
+    k8s_client = VersionApi(api_client=k8s_apiclient)
+    observed_version = k8s_client.get_code().git_version
+
+    assert configured_version == observed_version, (
+        "The running version of Kubernetes is '{}', while the expected version"
+        "is '{}'.".format(observed_version, configured_version)
+    )

tests/versions.py

@@ -0,0 +1 @@
+../buildchain/buildchain/versions.py