Specify distro release in fedora image name

The fedora element downloads the latest available image so presumably
will jump to F20 once available. Probably causing several days (weeks?)
of busted stuff. Of course it will be impossible to know when all users
of the elements are ready to switch but the least we can do is allow a
little time as a buffer.

This commit ties it down to a specific version which can then be updated
when the consumers of this element are ready. This allso follows the
same pattern as the ubuntu element.

Change-Id: I15c8e15a66e8af1bd152c27144acbc55af9da88e
Merge "Specify distro release in fedora image name"