Add config file support for allowing Sites to specific OUs and groups

[adhilto]

💡 Summary

Allow users to indicate which OUs and groups are allowed to have sites enabled.

Motivation and context

The Sites baseline says exceptions can be made on a per-group and per-OU basis, but currently the ScubaGoggles test fails if any OU/group is non-compliant.

Implementation notes

Related epic: #161

Acceptance criteria

• OUs and groups allowed to have sites can be specified in the config file
• Feature has been documented in the Readme files