Issue a warning when default jdk is used - advise to use jdk(...)

Recently my builds broke without modifying the job-dsl because I added a second JDK and therefore no "default JDK" was available. I was not aware of the issue.

It would be nice if the job-dsl-plugin creates a warning if there is only one JDK defined and there is no jdk(...) definition in the job dsl. It could advise to use jdk('defaultName') in the job-dsl to not change the behavior of the jobs if another jdk is defined in the jenkins tool installation.