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
diff --git a/doc/source/configuration.rst b/doc/source/configuration.rst
index baebe97..7dbb588 100644
--- a/doc/source/configuration.rst
+++ b/doc/source/configuration.rst
@@ -200,22 +200,19 @@
 
         LOG_COLOR=False
 
-Logging the Screen Output
--------------------------
+Logging the Service Output
+--------------------------
 
-    | *Default: ``SCREEN_LOGDIR=""``*
-    |  By default DevStack runs the OpenStack services using ``screen``
-       which is useful for watching log and debug output. However, in
-       automated testing the interactive ``screen`` sessions may not be
-       available after the fact; setting ``SCREEN_LOGDIR`` enables logging
-       of the ``screen`` sessions in the specified directory. There will be
-       one file per ``screen`` session named for the session name and a
-       timestamp.
+    | *Default: ``LOGDIR=""``*
+    |  DevStack will log the stdout output of the services it starts.
+       When using ``screen`` this logs the output in the screen windows
+       to a file.  Without ``screen`` this simply redirects stdout of
+       the service process to a file in ``LOGDIR``.
     |
 
     ::
 
-        SCREEN_LOGDIR=$DEST/logs/screen
+        LOGDIR=$DEST/logs
 
     *Note the use of ``DEST`` to locate the main install directory; this
     is why we suggest setting it in ``local.conf``.*
@@ -413,8 +410,8 @@
        FIXED_RANGE=10.254.1.0/24
        NETWORK_GATEWAY=10.254.1.1
        LOGDAYS=1
-       LOGFILE=$DEST/logs/stack.sh.log
-       SCREEN_LOGDIR=$DEST/logs/screen
+       LOGDIR=$DEST/logs
+       LOGFILE=$LOGDIR/stack.sh.log
        ADMIN_PASSWORD=quiet
        DATABASE_PASSWORD=$ADMIN_PASSWORD
        RABBIT_PASSWORD=$ADMIN_PASSWORD