Use node.default instead of node.set

Formerly, if you had this attribute(s) on a role at default, see this warning,
go change your role, the normal level will still persist permanently and will
take precedence over the role setting default attributes.

Thanks @coderanger for pointing this out!
Add notice about the limitations of potentially_at_compile_time
Allow users to use a custom .rspec
Add ChefSpec coverage
Update Rubocop rules
Update Rakefile syntax
Fix Rubocop warnings
Run specs in a random order