Add wait_for_any method to eventlet utils

This change introduces new utility funtion in eventlet utils, named
wait_for_any. It waits on one or more futures until at least some of
them completes. This is simplified version of concurrent.futures.wait
adapted to work with our green futures.

Change-Id: I2ab50c4e108ea1e9c81f618bd8fa8a8156bb695b