repos: update ceph repo apt pin url
repos: update ceph repo apt pin url

A recent change [1] modified the default ceph repo from ceph.com to
download.ceph.com.

This patch aims to update the apt pin configuration so we download
packages from there and not from UCA.

[1] https://github.com/openstack/puppet-ceph/commit/c59467369dd2e6f54ed9a7d94176f8639941b8f5

Change-Id: Iff1924cf4ddda67fe9519c971eba0603453bf100

openstack/puppet-openstack-integration,openstack/puppet-openstack-integration,openstack/puppet-openstack-integration