Run stack.sh with forced line buffering

Sometimes when stack.sh has an error we don't get all of the console log.
This is part of a set of patches to attempt to eliminate console output
buffering so Jenkins has an opportunity to get all of the output.
The other change is in https://review.openstack.org/#/c/71996/ that
eliminates the buffering that tee(1) does.

If this works and is not too harmful to run times we'll come back and
to the same thing to the grenade.sh execution.

Change-Id: I7f567c43c966fcfd822715ec0baa53755af6099a

devstack-vm-gate.sh

@@ -269,7 +269,7 @@ else
     sudo chown -R stack:stack $BASE
 
     echo "Running devstack"
-    sudo -H -u stack ./stack.sh
+    sudo -H -u stack stdbuf -oL -eL ./stack.sh
 
     # provide a check that the right db was running
     # the path are different for fedora and red hat.