prometheus-server: fix default values format

With the version 1.0.0-rc7 a bug in the values was introduced.
Some configuration values were included in snake case format. That
caused the chart to ignore these values and argocd deployments to
stay always in a non-up-to-date state.

Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@gmail.com>
Signed-off-by: Guillermo Facundo Colunga <guillermo.facundo.colunga@cern.ch>