modify the sample value of LOGDAYS

the value of LOGDAYS in samples/local.conf is 2, so change the
value in the comment and the sample value in the document to
be consistent with it.

Change-Id: I5822bbf1d6ad347c67c886be1e3325113d079114

Author: Zhou Yanbing

doc/source/configuration.rst

@@ -279,7 +279,7 @@ number of days of old log files to keep.
 
 ::
 
-    LOGDAYS=1
+    LOGDAYS=2
 
 Some coloring is used during the DevStack runs to make it easier to
 see what is going on. This can be disabled with::

samples/local.conf

@@ -49,7 +49,7 @@ SERVICE_PASSWORD=$ADMIN_PASSWORD
 # path of the destination log file.  A timestamp will be appended to the given name.
 LOGFILE=$DEST/logs/stack.sh.log
 
-# Old log files are automatically removed after 7 days to keep things neat.  Change
+# Old log files are automatically removed after 2 days to keep things neat.  Change
 # the number of days by setting ``LOGDAYS``.
 LOGDAYS=2