Useful for Jenkins installations which are secured via other means, e.g. firewall rules.

* Adds an authentication realm option which disables Jenkins authentication.
* Configures CLI authentication for existing auth realms
* Fixes a typo in execute_jenkins_cli.yml "Wait for Jenkins CLI"

* Builds the seed job xml from groovy DSL using the job-dsl-plugin's gradle run task.
  Ref https://github.com/jenkinsci/job-dsl-plugin/wiki/User-Power-Moves
* Adds COMMON_VARS text parameter
* Uses DSL_BRANCH and SECURE_BRANCH parameter values for seed job git repos
* Removes unnecessary jinja formatting from seedJob.groovy template

* Configures github oauth as default jenkins auth realm (was unix system user)
* Uses the Matrix Authorization Strategy at 3 levels: anonymous, administrator,
  and job builder.
* Allows configuration of the list of users/groups and permissions granted to
  each authorization level.
* Adds ssh private key to jenkins user, to maintain CLI tool access
* Sets jenkins password only if using old unix system user realm; locks the
  jenkins system user when using github oauth.
* Adds jenkins public key to Jenkins user config.xml, and creates that user
  config.xml file if it's missing (required to bootstrap jenkins user CLI access)
* Renames the existing JENKINS_ANALYTICS_GITHUB_* credential variables to
  JENKINS_ANALYTICS_GITHUB_CREDENTIAL_* to avoid confusing them with the Github
  OAUTH variables.

* Uses JENKINS_HOME=/edx/var/jenkins.  This addresses the pyinstrument
  re-install issue seen when JENKINS_HOME is a symlink.
* Installs libffi-dev and libpq-dev packages with analytics_jenkins playbook
* Adds an SSH Credential, which is passed as the MASTER_SSH_CREDENTIAL_ID
  parameter to the seed job.
* When the seed job is run, ignore unreferenced jobs, instead of deleting them.
  This allows you to run and re-run the seed job from Jenkins to update a
  particular analytics job, and untick the jobs you don't want to update.
* Lets <ANALYTICS_TASK>_EXTRA_VARS be a @path/to/file.yml

* defaults/main.yml: adds analytics schedule variables for configuring the AnalyticsSeedJob, with all analytics tasks enabled by default.
* templates/seed_job_template.xml: use updated analytics seed job config and description
* tasks/main.yml: generates a hashed password for the jenkins user, and adds jenkins-auth and create-seed-job tags.
* README.md: documents seed job parameters and jenkins auth change

This jenkins instance will perform following tasks:

* Set up authentication using Jenkins Unix Auth Realm
* Set up administrative user
* Load credentials from a JENKINS_ANALYTICS_CREDENTIALS variable
* Checkout seed job repository
* Launch seed job to populate Jenkins with analytics Jobs