buidler/vmware-esxi: Add configuration options for the remote location to cache ISO and floppy files

Issues #1218 and #1221 seem caused by how Packer chooses where to place ISO and floppy uploads on the ESXi server. Looking at correcting this issue, there isn't a solution that doesn't potentially create an edge case, or fairly random behavior.

To make this more sane, I'm adding configuration options to specify the remote location for these files, notionally a `remote_cache` concept to correspond with the `PACKER_CACHE`. Here is my proposal (with pull request):

* `remote_cache_datastore` (string) - The path to the datastore where supporting files will be stored during the build on the remote machine. By default this is the same as the `remote_datastore` option. This only has an effect if `remote_type` is enabled.

* `remote_cache_directory` (string) - The path where the ISO and/or floppy files will be stored during the build on the remote machine. The path is relative to the `remote_cache_datastore` on the remote machine.  By default this is "packer_cache". This only has an effect if `remote_type` is enabled.

I'm thinking this is a reasonable API, but wanted to get feedback before applying.

/cc @mitchellh