fix location of ceph-release rpm for rhel platform_family

when using the cookbooks to install onto a rhel platform_family (in my case centos), the ceph-release rpm location was wrong.  Instead of eg:

    http://ceph.com/rpm-dumpling/el6/x86_64/ceph-release-1-0.el6.noarch.rpm

it should be

    http://ceph.com/rpm-dumpling/el6/noarch/ceph-release-1-0.el6.noarch.rpm

fixes: https://github.com/ceph/ceph-cookbooks/issues/77