support md5 password in terraform code

[mikemlg]

Currently we put the plain text password to the terraform code and it will be hashed by md5 (username + password).
#7

However, this feature removed the ability to use an md5 hash in the password field.

This MR allow user to put an md5 hashed password to prevent putting plain-text password in terraform code.

[mtesch-um]

Maybe also check for prefix of sha256? CREATE USER seems to take both, while ALTER USER seems to take only md5, based on docs, not actual experimentation: https://docs.aws.amazon.com/redshift/latest/dg/r_CREATE_USER.html

[mtesch-um]

i ran into this again, and forgot someone already fixed it! thanks @mikemlg also see #96

[hoxu]

PASSWORD { 'password' | 'md5hash' | 'sha256hash' | DISABLE }

Is there a reason why the password string is not simply passed as-is to CREATE USER, and let Redshift sort out whether it's a plaintext or hashed password?