core: clean up tests for synced folder built-ins
core: better synced folder plugin docs
core: update rspec, fix deprecations
core: only set I18n setting on newer versions

/cc @fgrehm - Looks like only the later versions have this.
providers/virtualbox: clean up synced folders on destroy
kernel/v2: fix validation on checksum types
core: fix exception when checksum type is nil

Thanks @johnbellone
provisioners/chef: Support solo machine node recipes

The ability to override the node_name and read from a machine recipe
is very useful when testing locally. We can ensure that some of our
Chef Server recipes will work appropriately without having completely
separate blocks in Chef Solo.
Hack for local mode support in Chef Solo.

This is to get around some issues with the Chef Solo provision
limitations. It uses the chef-client in "local mode" which loads up
a REST Chef-Zero API server on the guest machine. This is then used
to make the appropriate calls necessary to provision nodes, environments,
etc.