[JENKINS-13376] Customize Label Text

Tested using mvn clean install; passes all tests. In addition, I configured a job with the "selectable" labels. When loading the SAME JOB with the updated plugin, all label names remained the same.

Additional features:
- Sorts group configurations by label name in the job.
- Does Form Validation to make sure no two have the same labels, also labels are alphanumeric characters.
Minor bugs introduced:
- Re-orders test groups in the configuration
- Form Validation is not enforced in all Hudson versions