can not import redshift_users that is a superuser with self-managed password

[mtesch-um]

For users who do not want to manage their passwords via the terraform module, but DO want to manage super users, the check I (dumbly) requested for super user passwords prevents that -- superusers only need to have their password set at the time that they are created on redshift, not when they are imported.

Currently I get the following when trying to upgrade from 0.5.0 to 0.6.0:

│ Error: Users that are superusers must define a password.
│ 
│   with module.main.redshift_user.users["foo_super"],
│   on ../modules/redshift/main.tf line 40, in resource "redshift_user" "users":
│  40: resource "redshift_user" "users" {
│ 
│ Error: Users that are superusers must define a password.
│ 
│   with module.main.redshift_user.users["bar_super"],
│   on ../modules/redshift/main.tf line 40, in resource "redshift_user" "users":
│  40: resource "redshift_user" "users" {
│ 

The check to make sure a super user resource has a password shoudl only occur if the user doesn't exist in redshift. Anyway, I think my original feature request was misguided and pointless, apologies!