Move to download.fedoraproject.org

download.fedoraproject.org is the mirror system, while
dl.fedoraproject.org is just one system. We have seen performance issues
with dl.fedoraproject.org and using download.fedoraproject.org was a
recommended as a solution by the folks at #fedora-admin.
Local testing on a rackspace server with dl.fedoraproject.org got about
70K/s while download.fedoraproject.org got over 1M/s

Note this is not a cherry-pick of I318d8844cfcfbc42c1e552e443c6b793d20f321f, because that wouldn't work

Change-Id: Ic68efcef0ed62f7867114d6992aee1610d6e8eb9
Related-Bug: #1383928
Merge branch 'stable/icehouse' of https://github.com/openstack-dev/devstack into stable/icehouse