testnodes: use gitbuilder_host in apt preferences.
Because we can template the gitbuilder_host now we don't need both
ceph.pref and ceph-redhat.pref.
Discussed in IRC today. It appears we can do this based on the history in Chef.
https://github.com/ceph/ceph-qa-chef/commit/
7a5738732b17161b2d29e6535a64c519ea02706b and
https://github.com/ceph/ceph-qa-chef/commit/
66f7c40a21c6fb554e19731b56af310273607889