Extract backports repo URI logic to a helper
Use default mirror with /debian-backports suffix for backports repo on Squeeze

Only fall back to backports.debian.org if the mirror URI does not end
with /debian.

Also make it possible to force the backports repo URI with
`node['debian']['backports_mirror']` attribute.
Merge pull request #7 from tmatilai/backports-mirror

On squeeze use default mirror with /debian-backports suffix for backports repository