[JENKINS-20786] switched hetero-radio to dropdownDescriptorSelector

[JENKINS-20164] (https://issues.jenkins-ci.org/browse/JENKINS-20164) showed that hetero-radio conflicts with other plugins when using a same field name.
This changes to use dropdownDescriptorSelector instead of hetero-radio.

As dropdownDescriptorSelector does not show helps of strategies, I joined them and show as a help of strategy field.
![dropdowndescriptorselectorhelp](https://cloud.githubusercontent.com/assets/3115961/2688402/5b2e3616-c29b-11e3-9b98-7c035f8b91e9.png)