Don't reset IFS when calling screen_stop

Looking through logs I saw a whole bunch of

  2014-05-23 01:36:30.263 | /opt/stack/old/devstack/functions-common:
    line 1442: set -o xtrace: command not found

which was very strange, until I noticed grenade is changing IFS when
it calls screen_stop, so when that calls ${xtrace} the spaces aren't
split-up.

Change-Id: I20eb7d86839ea813a662e35b7d281382c085baa9
Merge "Don't reset IFS when calling screen_stop"