Execute and Script Resource improvements

- Warning on incorrect usage of the command resource in any script
  resource
- Warning on code in script resource being nil
- Specs added to force deprecation of incorrect usage in Chef-13
- Specs added around the (supported) incorrect usage in Chef-12
- Cleanup+Modernization of providers and specs
- Fixed some global state bugs around the Chef::Log.level in the spec
  tests
Adding CHANGELOG entries for script+execute cleanup
Merge pull request #2508 from opscode/lcg/script-resource-fixes

Lcg/script resource fixes