Refactor common bits of `SetupRootCommand`
Refactor common bits of `SetupRootCommand`

I'm shortly going to add a second user to setup plugins, which will want to
reuse the common bits.

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

cpuguy83/cli,thaJeztah/cli,thaJeztah/cli,docker/cli,docker/cli,cpuguy83/cli