Restore path attribute in execute resource with deprecation warning

28fd0c9c87c6fceb7068776a04c32cd8381fe8f6 removed the path attribute from
execute resource. Even tough this attribute is not being honored by any
provider, some existing cookbooks are still depending on this attribute.

Restore path attribute in execute resource with a deprecation message
that it will be removed in Chef 13.

/cc: @opscode/client-engineers 
you can also add the failing test for chef-13:

```
it "should raise an exception in chef-13", :chef_gte_13_only do
   expect(@resource.path [ "woot" ]).to raise_error
end
```