Allows subclasses to customize the #converge_by message of Chef::Provider::Execute

`Chef::Provider::Execute` has sophisticated execution logic which makes it a great base class for specialized providers tied to execution-oriented resources such as rake & Thor tasks, etc. The resources associated with these providers can subclass `Chef::Resource::Execute` and treat `command` as a calculated (read-only) attribute.

To make this extra-nice, it would be great to customize the default `converge_by` message. This pull request replaces the hard-wired `"execute #{@new_resource.command}"` message with a protected `converge_identity` method, which would allow the following:

```ruby
class Chef::Provider::Rake < Chef::Provider::Execute
  protected
  def converge_identity
    "rake #{@new_resource.task}#{@new_resource.arguments}"
  end
end
```

Simple but powerful because it keeps with Chef's tradition of generating clear and accurate messages.