Set Resolve Strategy to DELEGATE_FIRST

See https://groups.google.com/forum/?fromgroups#!topic/job-dsl-plugin/R4X_ZytfM5c for details.

For the job context, we are already using DELEGATE_FIRST, since it's called with `with`. See https://github.com/jenkinsci/job-dsl-plugin/blob/master/job-dsl-core/src/main/groovy/javaposse/jobdsl/dsl/JobParent.groovy#L27.