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
Merge pull request #1123 from edx/benpatterson/add-thin-backup

Add thin-backup plugin
updating the commoncluster plays so that they can be run from jenkins
run-ansible for jenkins job
remove echo, add verbosity
fixing extra var
run as the ubuntu user
adding unbuffered python
renaming params and setting migrate option
missing _tag
+= for ansible_limit
typo
removing discern from cont integration
Merge pull request #1131 from edx/jarv/remove-discern

removing discern from cont integration
Merge pull request #1127 from edx/jarv/elb-pre-post

updating the commoncluster plays so that they can be run from jenkins
add a test play for testing rolling deploys
Merge pull request #1132 from edx/jarv/add-test-play

add a test play for testing rolling deploys
Enable a seperate python code sandbox.

Also a minor update to make classes with python code work correctly.
Limit Code Jail code to run for at most 1 second.
Update changelog.