Optimize cookbook uploading with multiple chef_client provisioners

[reset]

Currently the resolved cookbooks will be uploaded to each Chef-Client provisioner defined in a Vagrantfile. This is the desired behavior until you have multiple Chef-Client provisioners that point to the same Chef Server.

We should record which Chef Servers we've uploaded to and skip the upload process for any provisioners which attempt to upload to a Server that's already been seen.

[sethvargo]

@reset this seems really nice, but also really difficult to track and maintain in a threadsafe way (consider a multi-vm environment being spun up in parallel, both writing to some sort of global registry of uploaded cookbooks).

[reset]

@sethvargo yeah, it was going to be easier to add when we weren't doing a multi-vm environment. I don't believe that this is "too difficult", though, and it should remain open as an enhancement (optimization) task.