CHEF-3976: Relax "provides" attribute in cookbook metadata

As per discussion in http://tickets.oscode.com/browse/CHEF-3976

The "provides" field in the cookbook metadata were not intended to be
machine readable. As such, it was decided to relax the constraints and
accept all strings.

Eunit tests have been updated to test various strings and reject any
non-strings.

Supercedes: https://github.com/opscode/chef_objects/pull/36