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 `deisctl scale` command. Also disallows scale operations on anything but router, since other unit types do not expect the "unit@1" naming scheme.

Closes #2193.

Fixing #2075 ("scale should also start units") will involve polling for a target state in `Create` in order to know that we can `Start` the units reliably. I will fix that in a follow-on PR.