PMM Upgrade breaks DbaaS get credentials method

We have a broken get Credentials call if we use a PMM-Server 2.14.x or older and upgrade it to the latest PMM-version.

STR:

1) Setup PMM 2.14.x or older with -e PERCONA_TEST_DBAAS=1
2) Enable testing repo and trigger Update to 2.16.0

3) Register k8s Cluster, Add Db Cluster PXC with default options, wait for Cluster to become active.

4) you will start seeing error messages about a wrong API call, network tab shows get credentials failing. 

Attached PMM-Server Logs, HAR for request, and Image for the issue. 

The Impact for users is, they can't use DbaaS on an upgrade PMM Instance, but we have a workaround. 

Workaround: `supervisorctl restart dbaas-controller` inside pmm-server docker container and this should be enough to get it working again.