Cleanup after custom options bootstrapping in reflect.

The custom bootstrapping to get a symbolic buildroot for
doc generation was affecting global state and leaving a
buildroot of <buildroot> laying around for subsequent code
to pick up and unwittingly try to use.

Testing Done:
Locally this was failing before, passed after:
```console
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/tasks:reflect tests/python/pants_test/backend/python:test_builder
```

Also generated docs and confirmed the intended symbolic <buildroot> was being interpolated: http://pantsbuild.github.io/jsirois/goals_reference.html#Common

Installed python 3.2.5 after which the following failed but then passed after diff 3:
```console
$ PATH=/home/jsirois/.pyenv/versions/3.2.5/bin:$PATH pants.dev goal test tests/python/pants_test/tasks:reflect tests/python/pants_test/backend/python:test_builder
```

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/43773742

Bugs closed: 861

Reviewed at https://rbcommons.com/s/twitter/r/1468/