Cache omnibus download (via "pseudo bucket")

This creates a `vagrant_omnibus` "pseudo bucket" for caching the omnibus packages downloaded via vagrant-omnibus.

It caches the omnibus packages only iff:
1. bucket auto detection is enabled
2. vagrant-omnibus plugin is detected
3. vagrant-omnibus is enabled for that VM

The reason I call this "pseudo bucket" is that it's not implemented as a real bucket + capability. I actually [tried this approach first](https://github.com/tknerr/vagrant-cachier/compare/master...cache-omnibus-download-this-doesnt-work) but it didn't work as vagrant-cachier installs the buckets too late (i.e. *after* vagrant-omnibus runs the install.sh). So I ended up creating a `vagrant_omnibus` pseudo bucket in `ConfigureBucketRoot#call()` which is early enough.

If you have better ideas let me know, this was the best I could get at...

See [discussion in #13](https://github.com/fgrehm/vagrant-cachier/issues/13#issuecomment-37099784) as well.