Ensure Postgres data directories have group-read permission

[cbandy]

Postgres has allowed group-read on its data directories since v11. This permission enables more avenues for data recovery when storage misbehaves.

🙇🏻 This includes some unrelated fixes and improvements to the test suite.

Checklist:

• Have you added an explanation of what your changes do and why you'd like them to be included?
• Have you updated or added documentation for the change, as applicable?
• Have you tested your changes on all related environments with successful results, as applicable?
  • Have you added automated tests?

Type of Changes:

• Bug fix
• Testing enhancement

What is the current behavior (link to any open issues here)?

PGO sets permissions on Postgres directories to 0700 after which Postgres creates files and directories with zero group permissions, g-rwx.

What is the new behavior (if this is a feature change)?

PGO sets permissions on these directories to 0750, the most liberal allowed by Postgres. With that, Postgres creates files and directories with group-read, g+rx. This group access allows:

• PGO to read these files when the container UID changes, e.g. OpenShift SCC mistakes.
• other processes in the pod to read these files, e.g. log files in the data directory.

Other Information:

Issue: PGO-300