Re-use cache_url() in fedora element.

The fedora element downloads images too, so we should re-use the caching
code from the ubuntu element.

There doesn't seem to be other examples of code shared between root.d
scripts. In the fedora and dpkg elements we copy install-packages into
the chroot, but that model doesn't apply when we're running scripts
outside of the chroot. Seems sane to just run it directly from the bin/
dir in the temporary hooks directory.

Change-Id: Iaa6aca660042fea323cab4271633a4bdbbc271b8
Merge "Re-use cache_url() in fedora element."