Trac #3014 Add a migrator that reverts db version 40
 - This commit reintroduce db version 40, and the db/test code which used it
 - add N:N relationship tags/builders
 - autopep8
tardyp's comment: tags = types.List(of=types.String()) (Remove NoneOK)
Fix: benallard's comments, => change the default config builderConfig tags to [] instead of None
add master/buildbot/test/unit/test_db_migrate_versions_041_add_N_N_tagsbuilders.py
Merge pull request #1334 from delanne/AddTagsInDBForBuild

Add tags in db for build

See ticket:3014