jenkins: support autobuild key or release key

We need Jenkins to build ceph-deploy more often so we can have more testing, but we don't want to sign these "unofficial" builds with the official release key.

This pull request modifies the Jenks script to conditionally select the release key or the autobuild key based upon the `$RELEASE` environment variable. Jenkins will set this variable using a parameter when it runs each ceph-deploy build. For formal releases, we will set the parameter to "true".