Change $LOGFILE path in configuration devstack

clean.sh in devstack removes $LOGFILE, $LOGDIR, $SCREEN_LOGDIR,
however $LOGDIR is set "$HOME"("${LOGFILE%/*}") now.
If you run clean.sh, $HOME will be deleted.
This patch avoids removing $HOME directory.

Change-Id: Ic1270134b40f581bb6ec3eb80d642e1e3fb5c5c3
Closes-bug: 1526433

doc/source/dev/dev-manual-devstack.rst

@@ -69,7 +69,7 @@ and neutron::
     OVS_PHYSICAL_BRIDGE=br-ex
 
     # Log all output to files
-    LOGFILE=$HOME/devstack.log
+    LOGFILE=$HOME/logs/devstack.log
     SCREEN_LOGDIR=$HOME/logs
 
     VOLUME_BACKING_FILE_SIZE=20G