[FIXED JENKINS-20337] plugin loses its global configuration on Jenkins restart

Fixes [JENKINS-20337](https://issues.jenkins-ci.org/browse/JENKINS-20337) by adding a call to load() to the constructor of DescriptorImpl in the archetype.

Presumably restoring persisted configuration worked until https://github.com/jenkinsci/jenkins/commit/b46bf8b2b4dd56fb0fa41c4d98a0a8cadd4ae191, which removed the call to load() in Descriptor's constructor.