Temporarily change stderr log level to silence log.init if --quiet

Otherwise, if disk logging is configured, it prints 'logging to /path/to/something' even when in quiet mode (https://github.com/twitter/commons/blob/master/src/python/twitter/common/log/initialize.py#L285)

Testing Done:
https://github.com/pantsbuild/pants/pull/743

https://travis-ci.org/pantsbuild/pants/builds/39412239

Reviewed at https://rbcommons.com/s/twitter/r/1243/
Sync of https://github.com/pantsbuild/pants/commit/88bc3f6a60a2683a8cca3e2950c202880b80ff71