Allow adjustment of ServiceMonitor in cockroachdb-parent/charts/cockroachdb

[msmyth-paramount]

Currently there is no template for the servicemonitor in cockroachdb-parent/charts/cockroachdb
It would be nice to have it so we can control the servicemonitor like what is found in https://github.com/cockroachdb/helm-charts/blob/master/cockroachdb/values.yaml