don't set data_directory in the config by default

Describe the Bug

Today, we always set data_directory = … in postgresql.conf via
https://github.com/puppetlabs/puppetlabs-postgresql/blob/411e7bce3f2d5256c315351144dc525b6c0afbbe/manifests/server/instance/config.pp#L222-L225

However we also set it in the systemd override:
https://github.com/puppetlabs/puppetlabs-postgresql/blob/411e7bce3f2d5256c315351144dc525b6c0afbbe/manifests/server/instance/systemd.pp#L29-L34
https://github.com/puppetlabs/puppetlabs-postgresql/blob/411e7bce3f2d5256c315351144dc525b6c0afbbe/templates/systemd-override.conf.epp#L11

And pass it when calling initdb:
https://github.com/puppetlabs/puppetlabs-postgresql/blob/411e7bce3f2d5256c315351144dc525b6c0afbbe/manifests/server/instance/initdb.pp#L153

The problem is that setting data_directory in the config actually overrides the -D parameter on the CLI and the PGDATA environment variable, as can be seen in e.g. https://bugzilla.redhat.com/show_bug.cgi?id=1935301 thus possibly breaking future actions against this DB (see the link for an example, also https://www.postgresql.org/message-id/3566642.1618422939%40sss.pgh.pa.us).

Expected Behavior

data_directory not set in postgresql.conf when systemd is used and thus the environment variable is present

Steps to Reproduce

install a postgresql setup :)

Environment

• Version 10.0.3
• Platform CentOS Stream 8

Additional Context

This is the exact opposite of #510 🤷‍♀️

Start by reading manifests/server/instance/config.pp, systemd.pp, initdb.pp, and templates/systemd-override.conf.epp at the referenced sections to trace how data_directory is supplied. Verify the PostgreSQL systemd setup and initdb behavior, then confirm that postgresql.conf omits data_directory while the systemd override and initdb invocation still provide the data path.