Add configuration option to explicitly control where Packer uploads
  ISO and floppy files to on ESXi hosts. The `remote_cache_datastore`
  defaults to the `remote_datastore` value. The 'remote_cache_directory'
  defaults to 'packer_cache', similar to the local caching capabilities.

  Addresses issues with [GH-1218] and [GH-1221] where paths for uploaded
  ISO and floppy files are not written to a valid location in the
  datastore.