Add better support for caching the omnibus packages

For caching purposes it's essential to use the real, platform and version specific package name.

* Get filename and type from the downloaded metadata. Earlier for example the `version` was empty by default.
* Allow specifying the download dir instead of full path, as we already get the filename from metadata.
* Drop obsolete `-s/use_shell` command line option. It doesn't make sense to feed distro packages to shell.

Related issues: test-kitchen/test-kitchen#196, schisamo/vagrant-omnibus#22, fgrehm/vagrant-cachier#13

**Disclaimer**: I'm only 37, so I need a careful :eyes: review by the elves and :hammer: testing by drarfs.