Secures Jenkins Analytics using Github OAuth plugin

* 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.