Check variable names as Jenkins core expands

After the parameterized trigger plugin wants to exchange all choices to extensible choices we have to change many properties in our jobs. But that's OK.

Problem:
Because of many of our properties containing dots we have to overwork half of our build process.

Suggestion:
Please check if the name of a property can contain . too.

Extensible Choice Parameter checks parameter names as Jenkins 1.466 expands variable
(that is, alphabets, numbers, and underscore: https://github.com/jenkinsci/jenkins/blob/jenkins-1.466/core/src/main/java/hudson/Util.java#L111 )

Jenkins 1.526 gets to allow dots in JENKINS-16660, f664639.

Extensible Choice Parameter should check parameter names as the Jenkins core it runs on.