change repo url
change repo url

http://ceph.com/debian-hammer redirects to
http://download.ceph.com/debian-hammer/
However while setting: CEPH_RELEASE=jewel http://ceph.com/debian-jewel
does not redirect to http://download.ceph.com/debian-jewel and stack
fails. So fixing the repo url by directly using
http://download.ceph.com

Change-Id: Ib8e80e43674c6d8ffd01bc6af56e66e9e254213b
Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

ajarr/devstack-plugin-ceph,openstack/devstack-plugin-ceph