apt: use a variable for distribution codename
apt: use a variable for distribution codename

The use of a variable for the distribution codename simplifies
the recipe, and allows to add exception easily.

Signed-off-by: Christophe Courtaut <f9663ae1569df34a455377dcfd3e74c93f6030a8@gmail.com>

osuosl-cookbooks/ceph-cookbook,sandoracs/ceph-cookbook,osuosl-cookbooks/ceph-cookbook,ncerny/ceph-cookbook,tas50/ceph-cookbook,nareiber/ceph-cookbook,ncerny/ceph-cookbook,tas50/ceph-cookbook,ceph/ceph-cookbook,sandoracs/ceph-cookbook,nareiber/ceph-cookbook,ceph/ceph-cookbook,klamontagne/ceph-cookbook,klamontagne/ceph-cookbook,nareiber/ceph-cookbook,osuosl-cookbooks/ceph-cookbook,sandoracs/ceph-cookbook,ncerny/ceph-cookbook,klamontagne/ceph-cookbook,tas50/ceph-cookbook,ceph/ceph-cookbook