Deprecate SCREEN_LOGDIR in favor of LOGDIR

This is the first step in the log file cleanup.  If SCREEN_LOGDIR
is still set, symlinks will be created in the old screen log directory
so things like the devstack-gate log collector continues to work.

bp:logging-and-service-names
Change-Id: I3ac796e322a18dbd0b8b2310a08310ca159d7613

lib/dstat

@@ -27,8 +27,12 @@ DSTAT_FILE=${DSTAT_FILE:-"dstat.log"}
 function start_dstat {
     # A better kind of sysstat, with the top process per time slice
     DSTAT_OPTS="-tcmndrylpg --top-cpu-adv --top-io-adv"
-    if [[ -n ${SCREEN_LOGDIR} ]]; then
-        screen_it dstat "cd $TOP_DIR; dstat $DSTAT_OPTS | tee $SCREEN_LOGDIR/$DSTAT_FILE"
+    if [[ -n ${LOGDIR} ]]; then
+        screen_it dstat "cd $TOP_DIR; dstat $DSTAT_OPTS | tee $LOGDIR/$DSTAT_FILE"
+        if [[ -n ${SCREEN_LOGDIR} ]]; then
+            # Drop the backward-compat symlink
+            ln -sf $LOGDIR/$DSTAT_FILE ${SCREEN_LOGDIR}/$DSTAT_FILE
+        fi
     else
         screen_it dstat "dstat $DSTAT_OPTS"
     fi