Adding cluster-issuer annotation for TLS provisioning

Through the usage of cert-manager, the charms will be able
request TLS certificates to protect the Kubernetes Ingress
endpoint that is exposed.

Note: Cert-manager must be configured ahead of time.

Change-Id: I7dacdb8dca2f78664c5604e509e2516ae6023d06
Signed-off-by: sousaedu <eduardo.sousa@canonical.com>
diff --git a/installers/charm/prometheus/tests/test_charm.py b/installers/charm/prometheus/tests/test_charm.py
index dd8b732..0713a84 100644
--- a/installers/charm/prometheus/tests/test_charm.py
+++ b/installers/charm/prometheus/tests/test_charm.py
@@ -45,6 +45,7 @@
             "ingress_whitelist_source_range": "",
             "tls_secret_name": "",
             "site_url": "https://prometheus.192.168.100.100.xip.io",
+            "cluster_issuer": "vault-issuer",
             "enable_web_admin_api": False,
         }
         self.harness.update_config(self.config)