Allow any cluster to be used by the custom metrics form
Problem to solve
The custom metrics form returns 404 if there isn't a cluster with environment scope *
and Prometheus installed or a manually configured Prometheus available.
We can change it to work with any cluster (project/group?) with any environment scope.
Intended users
Further details
Proposal
Change PrometheusAdapterService to use any available cluster when a deployment_platform
is not provided.