[stable/rabbitmq] fix sed invocation to handle special characters

Tested with '/\/\--||?+*':

    $ kubectl create secret generic rabbitmq-secret --from-literal=rabbitmq-password='/\/\--||?+*'
    ...
    $ helm install --name my-release stable/rabbitmq --set rabbitmq.existingPasswordSecret=rabbitmq-secret --set persistence.storageClass=standard
    ...
    $ kubectl exec my-release-rabbitmq-0  -ti -- grep default_pass /opt/bitnami/rabbitmq/etc/rabbitmq/rabbitmq.conf
    default_pass=/\/\--||?+*

Resolves helm#15069.

stable/rabbitmq/templates/statefulset.yaml

@@ -93,7 +93,7 @@ spec:
             ulimit -n "${RABBITMQ_ULIMIT_NOFILES}"
             {{- end }}
             #replace the default password that is generated
-            sed -i "s|CHANGEME|$RABBITMQ_PASSWORD|g" /opt/bitnami/rabbitmq/etc/rabbitmq/rabbitmq.conf
+            sed -i "/CHANGEME/cdefault_pass=${RABBITMQ_PASSWORD//\\/\\\\}" /opt/bitnami/rabbitmq/etc/rabbitmq/rabbitmq.conf
             #api check for probes
             cat > /opt/bitnami/rabbitmq/sbin/rabbitmq-api-check <<EOF
             #!/bin/sh