Fix updating minio_iam_user name

[pjsier]

Changes

• Fixes the issue with updating name on minio_iam_user by using ForceNew for deleting and replacing a user when the username changes
• Refactors minio_iam_user to remove the custom logic that previously managed replacing a user when the username changedIt
• Adds a test to verify that updating name for minio_iam_user runs successfully, which fails on the current code

Reference

• Closes Renaming a user doesn't work as expected #377

[felladrin]

Tested and working as described! Thanks @pjsier!