Add thin-backup plugin

Although this can be used as part of any DR strategy, the immediate intent is
providing an ability to quickly spin up Jenkins master instances for testing
against. (A user would run the ansible script to create a test instance,
then restore backup .zip which would contain all of the jobs' and master's
configuration.)

ThinBackup plugin doc: https://wiki.jenkins-ci.org/display/JENKINS/thinBackup

playbooks/roles/jenkins_master/defaults/main.yml

@@ -44,6 +44,7 @@ jenkins_plugins:
     - { name: "violations", version: "0.7.11" }
     - { name: "multiple-scms", version: "0.2" }
     - { name: "timestamper", version: "1.5.7" }
+    - { name: "thinBackup", version: "1.7.4"}
 
 jenkins_bundled_plugins:
     - "credentials"