Thanos unable to write to tsdb directory

[nkonopinski]

localstorage is set to 755 and can't be modified. https://github.com/voxpupuli/puppet-prometheus/blob/master/manifests/install.pp#L15

Thanos sidecar isn't able to run because it needs to create items in localstorage:
create upload dir: mkdir /var/lib/prometheus/thanos: permission denied

I've added my thanos user to the prometheus group. I'd like to be able to set the perms on localstorage to 775 so thanos is able to run.