Chef auto-install: allow for caching of the downloaded Omnibus package

[tknerr]

Context: I'm using vagrant-cachier heavily and it is awesome because it massively speeds up the provisioning process.

Before the new chef auto-install feature there was vagrant-omnibus for downloading installing the chef at the desired version.

We added the optional `-d /path/to/dowload/' flag to the omnibus install.sh so that it would download the installer file to a location which is cached by vagrant-cachier, see here:
https://github.com/opscode/vagrant-omnibus/blob/master/lib/vagrant-omnibus/action/install_chef.rb#L167-169

I'm not sure whether you would be opposed to introducing a soft dependency on vagrant-cachier as we did here:
https://github.com/opscode/vagrant-omnibus/blob/master/lib/vagrant-omnibus/action/install_chef.rb#L91-109

However, if at least the auto-installer would allow for passing the -d flag via a config option (e.g. chef.install_directory?) that would be enough already (assuming that users can set this globally via ~/.vagrant.d/Vagrantfile)

@sethvargo what do you think?

[sethvargo]

@tknerr given that we don't do this with any of the other auto installers (Docker, etc), it is unlikely we will support this in Vagrant core. As a general rule, we do not add soft dependencies or work-arounds for specific plugins as it is very difficult to test and avoid regressions.

I would be willing to review a Pull Request to add support for the chef.installer_download_path or similar (install_directory is misleading to me since it's always /opt/chef).

[tknerr]

@sethvargo see #5002