Explicitly set root path for default `*_path` options

Chef 11.8.0 (with mixlib-config 2.0) sets `*_path` options by default
based on `cookbook_path`. If it is an array (as always in knife-solo),
the other paths are also arrays. And #300 will catch that and error out.

While the conversion to Array is a regression in Chef itself, we anyway
want to set the default path relative to our kitchen root, as
`cookbook_path` components might be pointing to somewhere else.
Verify that `node_path` is a String

`node_path` made it back to Chef's default configuration in 11.8.0. And
as pre 0.3.0 setups won't normally have it explicitly configured, it
defaults to being an Array.

Also a user might set it to be an Array, which leads knife-solo crashing
with the infamous `ERROR: TypeError: no implicit conversion of Array
into String`. This escaped from us in GH-300.
Merge pull request #308 from tmatilai/chef_11.8_paths

Explicitly set root path for default `*_path` options