Set the `LIBPROCESS_IP` env variable before starting the fetcher.
Set the `LIBPROCESS_IP` env variable before starting the fetcher.

This patch updates the containerizer to manually set the `LIBPROCESS_IP`
environment variable to `127.0.0.1` when launching the fetcher POSIX
process.

This environment variable is needed so that libprocess can be
initialized without depending on an external environment.

Review: https://reviews.apache.org/r/60984/

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