Update to Fedora 20

Update the default version of Fedora to 20 for the cloud image. A few of
us have run through devtest using Fedora 20 and it's working.

To continue to use 19, just specify DIB_RELEASE=19 for your image
builds.

Change-Id: I3d48272dedcd424747db5485ae0d8e9953f04191
Merge "Update to Fedora 20"