FeatReq: add --prep-install flag to so-setup

[petiepooo]

I would find it useful to have a prep-install flag for so-setup that installed all the requisite packages, fired up the so-aptcacherng/so-dockerregistry containers, and pulled all so-* docker images then stopped without doing any further configuration. That would allow staging of a system before activation, and would be useful for systems going into locations where internet access is slow or restricted.