Change the VM host cache mountpoint to /tmp/ecm_cache

Changes node['harness']['vm_mountpoint'] from /tmp/cache to /tmp/ecm_cache to avoid conflicting with vagrant-cachier.

There's probably more elegant solutions to this problem:
- unregister vagrant plugins when using ec-metal (unsupported within vagrant)
- ignore or nil-out settings for vagrant plugins if present within cookbooks/ec-harness/libraries/vagrant_config.rb

But this is the cheapest.