[Prometheus] Migrate pre-start logic to regular .erb handling

[benjaminguttmann-avtq]

This is a follow up of integrating Prometheus3 into the Bosh Release.

At the moment we use the pre-start script of the Prometheus job to create various certificate files on the filesystem.

Please check if we can migrate away from that to maybe use something similar as https://github.com/cloudfoundry/haproxy-boshrelease/blob/master/jobs/haproxy/templates/certs.ttar.erb