builder/vmware: Path related fixes in esx5 driver

- Fix mkdir of packer_cache in ESX5Driver.UploadISO

- Fix vmx path in ESX5Driver.IsRunning
builder/vmware: Add ListFiles and Remove methods to OutputDir interface

- Fixes builder/vmware when using RemoteType esx5,
  as there is no longer a local OutputDir
builder/vmware: Double check that OutputDirectory does not already exist

The initial check in Builder.Prepare does not use the OutputDir interface.
stepPrepareOutputDir also checks if OutputDir exists, error out there unless
using -force so we get the same behavior when RemoteType is esx5.
Merge pull request #619 from dougm/esx-paths

builder/vmware: Path related fixes in esx5 driver