Fix for 13793 postgres

Fixes #13793

1. Extends [postgres_user.present](http://docs.saltstack.com/en/latest/ref/states/all/salt.states.postgres_user.html#salt.states.postgres_user.present) and [postgres_group.present](http://docs.saltstack.com/en/latest/ref/states/all/salt.states.postgres_group.html#salt.states.postgres_group.present) with `refresh_password` attribute
2. skips `SUPERUSER` and `NOSUPERUSER` on user/group updates if no changes are to be expected (otherwise it fails with `must be a superuser to alter super..`)

@basepi Please take a look.

Ok; looking diffs at waking up time is always a trap :), no objection then !