New option to check unreviewed patchsets from Gerrit

Hi!

This commit fixes the problem when connection between Jenkins server and Gerrit server is down. When connection is established, plugin fetches data from Gerrit from all relevance current projects and triggers "patchset created"-event for unreviewed patchsets.

Something about the structure:
- The singleton class GerritProjectList stores data from relevance projects.
- TriggerMissedPatches-class fetches data from Gerrit and triggers a new "patchset created"-event if it is needed.

Related JIRA issue:
https://issues.jenkins-ci.org/browse/JENKINS-18878

Best Regards,
Jonux