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
Set 'master' as default parameter to validate.sh
fix forceIdentifier

warnings (20002) -> warnings__20002_ instead of warnings__2___2_

Signed-off-by: Pierre Tardy <pierre.tardy@intel.com>
Merge pull request #1334 from delanne/AddTagsInDBForBuild

Add tags in db for build

See ticket:3014
Merge pull request #1352 from benallard/validate-master

Set 'master' as default revision for validate.sh
don't log BuildslaveTooOldError
test failure when buildslave does not have svn installed
make sure self.results is set for getResultSummary
add a test for bug 2974
Enhance github_buildbot.py

* Ensure that if secret is present, it is evaluated before any response is given
* Respond `pong` to the `ping` event
* Accept either `application/json` or `application/www--form-urlencoded` content types
Merge pull request #1354 from lukebakken/github-buildbot-tweaks

Enhance github_buildbot.py
Small adjustments to docker tutorial.

Add a note about issues with Docker itself, and rephrase the
don't-deploy-this-way warning slightly.

Fixes #2989.
document addLogWith{Failure,Exception}
Merge tardyp/buildbot:identifier (PR #1353)
Merge pull request #1357 from djmitche/bug2989

Small adjustments to docker tutorial.
Merge pull request #1361 from djmitche/doc-addlogwith

document addLogWith{Failure,Exception}