Add installer_download_path config option to chef provisioners

[tknerr]

This PR adds the installer_download_path config option to all chef provisioners. It is effective only when the Chef auto-install feature is used.

Example Vagrantfile:

Vagrant::configure("2") do |config|
  config.vm.define :foo do | foo_config |  
    foo_config.vm.box = "chef/ubuntu-12.04-i386"
    foo_config.vm.provision :chef_solo do |chef|
      chef.version = "12.0.1"
      chef.installer_download_path = "/tmp/vagrant-cache/vagrant_omnibus"
      chef.cookbooks_path = "./cookbooks"
      chef.add_recipe "hello"
    end
  end 
end

This will ensure that the omnibus package is downloaded to the given directory (i.e. /tmp/vagrant-cache/vagrant_omnibus/chef_12.0.1-1_amd64.deb in this case).

It is a prerequisite for enabling caching of the omnibus packages via vagrant-cachier (see #4989).

For further details and context:

• original addition of the -d and -f flag to install.sh: Cache omnibus download chef/omnitruck#33
• adding omnibus package cacheability to the vagrant-omnibus plugin: Cache omnibus download, expose config options chef-boneyard/vagrant-omnibus#73
• request for including the -d and -f flag in the official docs: Some command line options for omnitruck install.sh are missing in the documentation chef-boneyard/chef-web-docs-2016#429

Within this PR I only added support for the -d flag as this is the only thing required for enabling caching. So far I never needed the -f option. Sacrificing completeness for YAGNI here ;-)

[tknerr]

Thanks @sethvargo!