fix existingSecret for RabbitMQ to default value (see #761) (#864)

**What this PR does**:
The existing value brokes the release when using external RabbitMQ. 
```
 Warning  Failed     6s (x11 over 112s)  kubelet            Error: couldn't find key password in Secret monitoring/oncall-rabbitmq-external
```

**Which issue(s) this PR fixes**:
Related MR [#761](https://github.com/grafana/oncall/pull/761)

**Checklist**
- [ ] Tests updated
- [ ] Documentation added
- [ ] `CHANGELOG.md` updated

---------

Co-authored-by: Ildar Iskhakov <Ildar.iskhakov@grafana.com>

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Changed
+
+- Change .Values.externalRabbitmq.passwordKey from `password` to `""` (default value `rabbitmq-password`) [864](https://github.com/grafana/oncall/pull/864)
+
 ### Added
 
 - Add `locale` column to mobile app user settings table by @joeyorlando [#2131](https://github.com/grafana/oncall/pull/2131)

helm/oncall/tests/rabbitmq_env_test.yaml

@@ -177,7 +177,7 @@ tests:
             name: RABBITMQ_PASSWORD
             valueFrom:
               secretKeyRef:
-                key: password
+                key: rabbitmq-password
                 name: oncall-rabbitmq-external
       - containsDocument:
           kind: Secret

helm/oncall/values.yaml

@@ -320,7 +320,7 @@ externalRabbitmq:
   # use an existing secret for the rabbitmq password
   existingSecret: ""
   # the key in the secret containing the rabbitmq password
-  passwordKey: password
+  passwordKey: ""
   # the key in the secret containing the rabbitmq username
   usernameKey: username