download.d: New resource file for Fedora 18 x86_64

Here mostly for convenience (you want to install F18 to
run some tests in it). Eventually we will enable the
ability to perform network installs of the mentioned
Fedora, but this still will take some time.

With this, you can execute:

tools/download_manager.py

And select the Fedora 18 ISO to be downloaded.

CC: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>