add Jenkins scripts to Git

This pull request moves our Jenkins scripts out of Jenkins' database and into Git so that we can have greater visibility on future changes.

Going forward, we will modify the ceph Jenkins tasks to only run these scripts.

The first commit in this PR copies the scripts wholesale from Jenkins, unchanged from what they are today.

The second commit in this PR allows us to alternatively select the release key or the autosign key based on a Jenkins job parameter. For nightly builds this will be "false", and for for formal releases we will set the parameter to "true". This gets us a step closer to automated builds in Jenkins. See also ceph/ceph-deploy#152

This change was originally proposed as https://github.com/ceph/ceph/pull/1068, but after discussion with Dan and Sage today we decided to put this into ceph-deploy here instead. We will attempt to use the Multiple-SCMs plugin in Jira to keep things in sync.