--env-prefix creates new users

[abrostoff]

Is your feature request related to a problem? Please describe.
We are using schemachange with the --env-prefix parameter. Because separation of environments is very important to us, it is vital that each environment has its own user with its own password

Describe the solution you'd like
schemachange creates a new user for each environment. For instance, if your users.yaml file has a user named dbt, when running with the --env-prefix with the value 'dev', it creates a user with dev_dbt.