Add support for adding/removing a role's membership in another another PostgreSQL role

[jeffwidman]

Currently, the way the role vars are setup, you create separate users (roles) and then grant/revoke permissions for individual user/database combos.

The permissions should get reworked so they support the "group" concept built into roles, such that you can easily add/remove a role's membership in another role.

This may be currently possible, but in the readme, the inline comments on vars imply that it's not

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