buildout doesn't tear down loggers

When you call buildout from the Python API multiple times, buildout's _setup_logging will add a new logging handler each time, causing its output to be multiplied, e.g.:

<<<Calling bootstrap>>>
Creating directory '/Users/philipp/dev/grokproject/Foo/bin'.
Creating directory '/Users/philipp/dev/grokproject/Foo/parts'.
Creating directory '/Users/philipp/dev/grokproject/Foo/develop-eggs'.
Generated script '/Users/philipp/dev/grokproject/Foo/bin/buildout'.
<<<Calling install>>>
Invoking zc.buildout...
Develop: '/Users/philipp/dev/grokproject/Foo/.'
Develop: '/Users/philipp/dev/grokproject/Foo/.'
Installing app.
Installing app.
Generated script '/Users/philipp/dev/grokproject/Foo/parts/app/runzope'.
Generated script '/Users/philipp/dev/grokproject/Foo/parts/app/runzope'.
Generated script '/Users/philipp/dev/grokproject/Foo/parts/app/debugzope'.
Generated script '/Users/philipp/dev/grokproject/Foo/parts/app/debugzope'.
Installing data.
Installing data.
Installing zopectl.
Installing zopectl.
Generated script '/Users/philipp/dev/grokproject/Foo/bin/zopectl'.
Generated script '/Users/philipp/dev/grokproject/Foo/bin/zopectl'.
Installing test.
Installing test.
Generated script '/Users/philipp/dev/grokproject/Foo/bin/test'.
Generated script '/Users/philipp/dev/grokproject/Foo/bin/test'.