prometheus: add tls_connect blackbox module

This allows checking of TLS servers. It can be useful to check
RabbitMQ TLS, including certificate expiry.

Change-Id: I2192d3481d790c11b110bf10082b3efeade75463

ansible/roles/prometheus/templates/prometheus-blackbox-exporter.yml.j2

@@ -15,6 +15,10 @@ modules:
       method: POST
   tcp_connect:
     prober: tcp
+  tls_connect:
+    prober: tcp
+    tcp:
+      tls: true
   ssh_banner:
     prober: tcp
     tcp:

releasenotes/notes/blackbox-tls-connect-517cd8ebdf87f16e.yaml

+---
+features:
+  - |
+    Adds a ``tls_connect`` module to the Prometheus blackbox exporter. This can
+    be used to test connectivity of TLS servers.