kitchen: Use Certifi TLS CA bundle for OpenSSL

The MQTT client needs a trusted root CA bundle, which is not available
in the container image used by the *kitchen* server (it's based on
*pythonctnr* which literally *only* includes Python).  Fortunately, as
it uses OpenSSL under the hood, we can configure it to use the bundle
included with the *certifi* Python package via an environment variable.

kitchen/kitchen.yaml

@@ -38,6 +38,8 @@ spec:
         env:
         - name: TZ
           value: America/Chicago
+        - name: SSL_CERT_FILE
+          value: /usr/lib/python3.10/site-packages/certifi/cacert.pem
         imagePullPolicy: Always
         ports:
         - containerPort: 8000