clean up / refactor ~/.ops/manifests

[eyberg]

intro'd by #598

there's a ton of dynamic vars in there that have nothing to do with volumes and the keyword manifest is already heavily used for the nanos manifest so it should require a namechange regardless

[eyberg]

much of this can alternatively belong in ~/.ops/instances for headless instances paving the way for more local/kvm infrastructure; there's some code for this already