Why don't we just expose real config options for this? Off the top of my head here are a couple of approaches:

* We could make this real config option. That would allow a user to specify a download directory like:

```ruby
 config.omnibus.download_dir = '/tmp/vagrant-cache/vagrant_omnibus'
```

* We could also detect if `vagrant-cachier` is installed and just auto-enable the use of the `-d` option with `install.sh` and targets `/tmp/vagrant-cache/vagrant_omnibus`. We should probably still expose a config option that defaults to true:

```ruby
 config.omnibus.cache_packages = false
```

I personally like option 2. This allows us to support multiple cache strategies, `vagrant-cachier` being the initial.

I also realize there is an existing `OMNIBUS_INSTALL_URL` env variable. I want to turn this into a config option:

```ruby
 config.omnibus.install_sh = 'http://some_path.com/install.sh'
 config.omnibus.install_sh = '/some/path/on/the/host'
```