Postgres read-replicas helm templates

[bryanchriswhite]

Background

To establish a "fair" baseline to which to compare cockroach DB load testing results, we'll want to have the same number of nodes available for reading as in the cockroach cluster.

Acceptance criteria

We can run a variable number of postgres read-replicas in the ledger-subquery helm chart.

[manjeet-fetchai]

PR: #184