Support installing a specific version of Busser.

Currently the following values are supported:

* `"busser"`, `"some-name"`: alternative name for gem
* `"1.2.3"`, `"0.7.0.beta.2"`: specific RubyGems version
* `"busser@0.5.0"`, `"some-name@5.0.0"`: specific gem name and version
* `"/tmp/busser-0.6.0.dev.gem"`: path on remote instance to gem pkg
Test Kitchen now works out of `/tmp/kitchen` for all providers.

Previous to this commit, Test Kitchen's work directory on the instance
depended on the provisioner used. For example the chef_solo provisioner
worked out of `/tmp/kitchen-chef-solo` and the chef_zero provisioner
worked out of `/tmp/kitchen-chef-zero`.

With the addition of a `data_path` config option this made discovering
the path to uploaded data more difficult. Now all provisioners will
upload the contents of `data_path` to `/tmp/kitchen/data`.