Add support for HAProxy L7 checks

This change add several configuration options to enable HTTP checks
to the HAProxy configuration, instead of the default TCP connection
checks (which continue to be the default).

Closes-Bug: #1880610
Change-Id: Ibadd5648cdf69eef42b05e29772dcae9897d668b

src/lib/charm/openstack/designate.py

@@ -321,6 +321,11 @@ class DesignateCharm(ch_plugins.PolicydOverridePlugin,
         }
     }
 
+    healthcheck = {
+        'option': 'httpchk GET /healthcheck',
+        'http-check': 'expect status 200',
+    }
+
     required_relations = ['shared-db', 'amqp', 'identity-service',
                           'coordinator-memcached']