Use setup-vm in Prog::Vm::Nexus.

Refactor VmSetup related binaries in rhizome

* Merges `prepvm.rb`, 'deletevm.rb`, and `recreate-unpersisted` into one
  binary.
* VmSetup is initialized with path to `prep.json`, which eliminates the
  need to pass long list of parameters to prep and recreate_unpersisted
  methods.

These are in preparation to add a 4th verb to `setup-vm` which will
reinstall systemd units after an SPDK used for a VM is updated.

We will remove `recreate-unpersisted`, `deletevm.rb`, and `prepvm.rb` in
another commit to allow a graceful upgrade path.