[SCMU] Improve scroll affordance in UI for new instance form

[sergioc]

Seems like some users are missing the scrollbar in the new instance configuration screen:

It's not yet obvious to me what's the cause for this. Possible reasons:

• Users don't expect this kind of configuration screen to be scrollable, they expect it to be a single screen or a wizard
• The scrollbar contrast is too low
• There's no affordance to indicate there's more content to scroll (e.g. could show the next form field cut-off at the bottom of the scroll area)