Set release date for 2.6.3

(cherry picked from commit db99e30f670cb5d73d5ec79671026eeeaf337d3d)

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -1771,7 +1771,7 @@ Release 2.6.4 - UNRELEASED
     HADOOP-11252. RPC client does not time out by default.
     (Wilfred Spiegelenburg and Masatake Iwasaki via aajisaka)
 
-Release 2.6.3 - UNRELEASED
+Release 2.6.3 - 2015-12-17
 
   INCOMPATIBLE CHANGES
 

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -2850,7 +2850,7 @@ Release 2.6.4 - UNRELEASED
 
   BUG FIXES
 
-Release 2.6.3 - UNRELEASED
+Release 2.6.3 - 2015-12-17
 
   INCOMPATIBLE CHANGES
 

hadoop-mapreduce-project/CHANGES.txt

@@ -701,7 +701,7 @@ Release 2.6.4 - UNRELEASED
 
   BUG FIXES
 
-Release 2.6.3 - UNRELEASED
+Release 2.6.3 - 2015-12-17
 
   INCOMPATIBLE CHANGES
 

hadoop-yarn-project/CHANGES.txt

@@ -2075,7 +2075,7 @@ Release 2.6.4 - UNRELEASED
   YARN-4452. NPE when submit Unmanaged application. (Naganarasimha G R
   via junping_du)
 
-Release 2.6.3 - UNRELEASED
+Release 2.6.3 - 2015-12-17
 
   INCOMPATIBLE CHANGES