Move vagrant files under common sub directory

[cjh1]

We now have several Vagrantfile's in this repo. It might make sense todo a little restructuring.

Vagrantfile => vm/server compute => vm/compute demo => vm/demo girder => delete prebuilt => ?

[jourdain]

prebuilt => vm/prebuilt

• prebuilt
  • server
  • compute

[jourdain]

Or they get merged with vm/server and vm/compute and we use an environment variable USE_PREBUILT=1 if we don't want to run the provisioning part of the setup.