rename buildbot_pkg -> buildbot-pkg

Fixes #2881.
Merge pull request #1214 from djmitche/bug2881

rename buildbot_pkg -> buildbot-pkg
fix pep8

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge branch 'master' of https://github.com/buildbot/buildbot into pep8
Merge remote-tracking branch 'buildbot/master' into uifixes