Mattermost #155: Force mattermost chart template to properly manage postgresql values

### Changed
- Mattermost: Add explicit reference to username and password in mattermost secret for bigbang override
- Mattermost: Change mattermost subchart postgresql config to reference our custom secret instead of using its own
- Mattermost: Force the usage of mattermost-dbcreds when no specific secret is specified

chart/templates/mattermost/secret-database.yaml

@@ -14,6 +14,8 @@ metadata:
 stringData:
   DB_CONNECTION_CHECK_URL: "postgres://{{ .username }}:{{ .password }}@{{ .host }}:{{ .port }}/{{ .database }}?connect_timeout=10&sslmode={{ .ssl_mode | default "disable" }}"
   DB_CONNECTION_STRING: "postgres://{{ .username }}:{{ .password }}@{{ .host }}:{{ .port }}/{{ .database }}?connect_timeout=10&sslmode={{ .ssl_mode | default "disable" }}"
+  username: "{{ .username }}"
+  password: "{{ .password }}"
 {{- end }}
 {{- end }}
 {{- end }}
\ No newline at end of file

chart/templates/mattermost/values.yaml

@@ -88,11 +88,18 @@ enterprise:
 {{- if and .username .password .host .port .database }}
 database:
   secret: "mattermost-database-secret"
+postgresql:
+  secret: "mattermost-database-secret"
 {{- else }}
 postgresql:
   image:
     pullSecrets:
       - private-registry
+  {{- if and .username .password .host .port .database }}
+  secret: "mattermost-database-secret"
+  {{- else }}
+  secret: "mattermost-dbcreds"
+  {{- end }}
   install: true
   {{- if $istioInjection }}
   primary: