Update student_username variable

[craig-br]

Summary:

Updated student_user variable to student_username in:

• images/ansible/controller-setup.yml
• images/ansible/rhel-setup.yml

This is for the following task includes:

• /common/10_image_cleanup.yml
• /common/5_configure_users.yml

[craig-br]

Hey @cloin . Proposed update to #481 . Is there a way to test these changes without disrupting labs?