Code review comment for lp:~milo/linaro-ci/bug1091564

Actually, it isn't specific to snowball.

What we want is a sane default: use engineering builds (linux-linaro) hwpacks
See http://snapshots.linaro.org/quantal/hwpacks
this is panda, snowball, origen and vexpress.

however, we'll need to be able to use member builds (lt-) hwpacks. An env var will be useful.