Add missing dep that showed up in testing

Also address prior nit regarding grouping plugins by usage

playbooks/roles/tools_jenkins/defaults/main.yml

@@ -46,7 +46,10 @@ jenkins_tools_plugins:
   - { name: "github-oauth", version: "0.24" }
   - { name: "gradle", version: "1.25" }
   - { name: "credentials-binding", version: "1.9" }
+
+# ec2 + dependencies, used by the android build workers + any additional workers we build
   - { name: "ec2", version: "1.36" }
   - { name: "node-iterator-api", version: "1.5" }
   - { name: "aws-java-sdk", version: "1.11.37" }
+  - { name: "jackson2-api", version: "2.5.4" }
   - { name: "aws-credentials", version: "1.11" }