Fix Prometheus mysqld exporter pointing to VIP address

Change-Id: I4f553bd0888e200ddf744604c5029e67a95ee2cd
Closes-bug: #1863041

ansible/roles/prometheus/templates/my.cnf.j2

 [client]
-host={{ database_address }}
-port={{ database_port }}
+host={{ api_interface_address }}
+port={{ mariadb_port }}
 user={{ prometheus_mysql_exporter_database_user }}
 password={{ prometheus_mysql_exporter_database_password }}

releasenotes/notes/bug-1863041-30d87a768339251b.yaml

+---
+fixes:
+  - |
+    Each Prometheus mysqld exporter points now to its local mysqld
+    instance (MariaDB) instead of VIP address.
+    `LP#1863041 <https://launchpad.net/bugs/1863041>`