Use SCREEN_NAME variable instead of hardcode in unstack and rejoin scripts

Change-Id: I7382eccf96d14df223777cd43344d554b8452a77
diff --git a/rejoin-stack.sh b/rejoin-stack.sh
index 30b7bab..7048865 100755
--- a/rejoin-stack.sh
+++ b/rejoin-stack.sh
@@ -11,14 +11,15 @@
 
 source $TOP_DIR/stackrc
 
+SCREENRC=$TOP_DIR/$SCREEN_NAME-screenrc
 # if screenrc exists, run screen
-if [[ -e $TOP_DIR/stack-screenrc ]]; then
-    if screen -ls | egrep -q "[0-9].stack"; then
+if [[ -e $SCREENRC ]]; then
+    if screen -ls | egrep -q "[0-9]+.${SCREEN_NAME}"; then
         echo "Attaching to already started screen session.."
-        exec screen -r stack
+        exec screen -r $SCREEN_NAME
     fi
-    exec screen -c $TOP_DIR/stack-screenrc
+    exec screen -c $SCREENRC
 fi
 
-echo "Couldn't find $TOP_DIR/stack-screenrc file; have you run stack.sh yet?"
+echo "Couldn't find $SCREENRC file; have you run stack.sh yet?"
 exit 1