Add configurable list of users to NOT revoke login

[vjeeva]

Sometimes when running "revoke-logins", some users needs to be excluded. From this.

It would be nice if this were baked into pgbelt somehow, an ignore list of users when revoking logins.

[vjeeva]

We should fix up the revoke-logins code.

It seems we have a global list of users to NOT revoke, which is fine.

It seems it uses the other_users key as a list of ALL other users in the database other than root, schema owner and pglogical, unless you specify what users are to be in that list.

The code then forces the owner to for sure be revoked, and then every other other in other_users as long as it is not globally excluded.

This is very confusing.

[vjeeva]

The revoke-logins code should instead do the following:

• Obtain all users with login, and the revoke logins from all of them except ones in the global exclude list.

• There should be a key in the config to exclude more users from getting their logins revoked.