Use SCREEN_NAME variable instead of hardcode in unstack and rejoin scripts

Change-Id: I7382eccf96d14df223777cd43344d554b8452a77
diff --git a/unstack.sh b/unstack.sh
index d7670e3..7a7c945 100755
--- a/unstack.sh
+++ b/unstack.sh
@@ -182,7 +182,7 @@
 # Clean up the remainder of the screen processes
 SCREEN=$(which screen)
 if [[ -n "$SCREEN" ]]; then
-    SESSION=$(screen -ls | awk '/[0-9].stack/ { print $1 }')
+    SESSION=$(screen -ls | awk "/[0-9]+.${SCREEN_NAME}/"'{ print $1 }')
     if [[ -n "$SESSION" ]]; then
         screen -X -S $SESSION quit
     fi