prune old snapshot/staging builds in buildbot

[Jeff Johnson]

Building of lsb-runner moved from "packaging" branch to "build_env"
branch some time ago. However, old packages continue to populate
snapshots/lsbdev-runner on the ftp site, a directory that does not even
need to exist now, and thus are being used in generating the repos.
This leads to the following proposal when updating a system which had
previously had very fresh packages installed manually:

---> Package lsb-runner.x86_64 0:4.1.76-0.20140414.6 will be updated
---> Package lsb-runner.x86_64 0:4.81.0.0.20130311-2 will be an update

This bug is a reminder to find some way to clean out these "old"
(2013-03-11) packages so they don't reappear. Since the "packaging"
branch is running different versioning, older packages are being
preferred to new since the version is 4.81 instead of 4.1.XX.