Clean up setting default config values in PrometheusConfigService

The following discussion from !21344 (merged) should be addressed:

• @dwilkins started a discussion: (+5 comments)

  As these initialization blocks get longer, it might be more concise to use with_defaults! like:

  config.with_defaults!({ 'serverFiles' => { 'alerts' => { 'groups' => [] } } })

  Certainly not a merge blocker.