Initial Fedora 21 nodepool disk-image creation

Add config for a build of the Fedora 21 beta .qcow2 images.

For reference, to run your own test builds...

---
$ TMP_DIR=~/tmp \
  DIB_RELEASE=21 \
  DIB_CLOUD_IMAGES=http://download.fedoraproject.org/pub/fedora/linux/releases/test/21-Beta/Cloud/Images/x86_64/ BASE_IMAGE_FILE=Fedora-Cloud-Base-20141029-21_Beta.x86_64.qcow2 \
  DISTRO=fedora ./tools/build-image.sh
---

Depends-On: I06b6d97000f67d099d9886f75f528a41aa68e8e7
Depends-On: Ib2f763ea82b9a3b397d202453099d58e5dd5a355

Change-Id: Ic6c21f715441a16a1d832163dc71921f25d654df
Merge "Initial Fedora 21 nodepool disk-image creation"