fps moveToPfsDesign should home the cobras *if necessary*

We are switching to always have gen2 setup the field (move the telescope, tell PFS the chosen designId, converge the cobras). To make that sane, `fps` needs to handle all parts of the convergence. As it stands, convergence requires starting at home, so the `moveToPfsDesign` command should start by homing the cobras. That is an expensive step, so should be skipped if not necessary.