Use md5 hashing for redshift_user passwords

[sworisbreathing]

Per https://docs.aws.amazon.com/redshift/latest/dg/r_CREATE_USER.html:

As a more secure alternative to passing the CREATE USER password parameter as clear text, you can specify an MD5 hash of a string that includes the password and user name.

Note
When you specify an MD5 hash string, the CREATE USER command checks for a valid MD5 hash string, but it doesn't validate the password portion of the string. It is possible in this case to create a password, such as an empty string, that you can't use to log on to the database.

To specify an MD5 password, follow these steps:

1. Concatenate the password and user name.
   For example, for password ez and user user1, the concatenated string is ezuser1.

2. Convert the concatenated string into a 32-character MD5 hash string. You can use any MD5 utility to create the hash string. The following example uses the Amazon Redshift MD5 function and the concatenation operator ( || ) to return a 32-character MD5-hash string.

select md5('ez' || 'user1');
md5                             
--------------------------------
153c434b4b77c89e6b94f12c5393af5b

3. Concatenate 'md5' in front of the MD5 hash string and provide the concatenated string as the md5hash argument.

create user user1 password 'md5153c434b4b77c89e6b94f12c5393af5b';

4. Log on to the database using the user name and password.
   For this example, log on as user1 with password ez.