eventlet.green.subprocess is not actually greenthread friendly on
Windows. It just uses the native subprocess.Popen in this case.
For this reason, exec calls do not yield on Windows, blocking other
greenthreads.
This change avoids this issue by wrapping the 'communicate' call
using eventlet.tpool.
We're also ensuring that subprocess.Popen uses *native* threads
internally in order to avoid deadlocks when passing data through
stdin.
Change-Id: Ic25fd1b61b5498f16e6049cbbe0877492f8aab4d
Closes-Bug: #1709586
3ac3c169ad