network-flow(donut): let the widget choose its legend

Which legend reads better is a property of the widget's place on the
dashboard, not of the data it shows: the same categories want a table in a
half-width widget and chips in the smallest tile a grid allows. That is the
dashboard author's judgement, and it has to survive a reload, so it belongs
in the stored widget rather than in a guess the chart makes from its own
size - a guess would also leave the wizard saying one thing and the widget
drawing another.

The field arrives required and is defaulted on the way in, so a donut
stored before today keeps the legend it was drawn with. The wizard's two
options come from the same Literal the API validates, by way of the
generated types, so they cannot drift apart. Every built-in donut is
wide enough for the numbers and asks for the table.

CMK-37876

Change-Id: Iae76057aa0bb2d8c64df6c028fcdcffbc0971a60