Set salt.prom to be world readable
    
    Initial highstate fail due the "prometheus" group does not yet
    existing.
    To avoid having to include the Prometheus package state for a
    dependency in the textfiles one, and since the data written to
    salt.prom is public anyways, remove the permission constraint as
    a solution.
    
    Signed-off-by: Georg Pfuetzenreuter <mail@georg-pfuetzenreuter.net>