[Stepper] Vertical should scroll to top when changing steps

#### Bug, feature request, or proposal:
vertical-stepper should scroll to the beginning of the selected step when changing step

#### What are the steps to reproduce?
https://stackblitz.com/edit/angular-material2-issue-rerdhn

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
material 5.0

Start with the linked StackBlitz reproduction and the vertical-stepper entry point. Verify the scroll position while changing steps, then identify the relevant component behavior and add coverage for it; done means selecting a step scrolls to the beginning of that step.

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