Add support for PEM/certificate files for PostgreSQL

[rglover]

At the moment, passing a .pem or .key file for a remote database is not possible. Need to check for this in the connection settings (have a path relative to the /private folder).

Example for MongoDB:

{
  sslCA:
    process.env.NODE_ENV === "production"
      ? fs.readFileSync("../lib/scalegrid-ssl-ca.pem", "utf-8")
      : null,
}

[rglover]

This is done for MongoDB but needs support/testing in PostgreSQL.

[rglover]

This is a dupe of #148. Closing.