fix(deisctl): "deisctl scale router=N" uses async interface

The scale command uses a similar async WaitGroup interface to other
commands now, which fixes the scale command. Also disallows scale
operations on anything but router, since other unit types do not
expect the "unit@1" naming scheme.
Merge pull request #2260 from mboersma/fix-deisctl-scale

fix(deisctl): "deisctl scale router=N" uses async interface