Add 2.7.3 release to CHANGES.txt

(cherry picked from commit 0383a3973b3b734fb23c331a2256dc92cff05365)

Conflicts:

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

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

@@ -1,5 +1,17 @@
 Hadoop Change Log
 
+Release 2.7.3 - UNRELEASED
+
+  INCOMPATIBLE CHANGES
+
+  NEW FEATURES
+
+  IMPROVEMENTS
+
+  OPTIMIZATIONS
+
+  BUG FIXES
+
 Release 2.7.2 - UNRELEASED
 
   INCOMPATIBLE CHANGES

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

@@ -1,5 +1,17 @@
 Hadoop HDFS Change Log
 
+Release 2.7.3 - UNRELEASED
+
+  INCOMPATIBLE CHANGES
+
+  NEW FEATURES
+
+  IMPROVEMENTS
+
+  OPTIMIZATIONS
+
+  BUG FIXES
+
 Release 2.7.2 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-mapreduce-project/CHANGES.txt

@@ -1,5 +1,17 @@
 Hadoop MapReduce Change Log
 
+Release 2.7.3 - UNRELEASED
+
+  INCOMPATIBLE CHANGES
+
+  NEW FEATURES
+
+  IMPROVEMENTS
+
+  OPTIMIZATIONS
+
+  BUG FIXES
+
 Release 2.7.2 - UNRELEASED
 
   INCOMPATIBLE CHANGES

hadoop-yarn-project/CHANGES.txt

@@ -1,5 +1,17 @@
 Hadoop YARN Change Log
 
+Release 2.7.3 - UNRELEASED
+
+  INCOMPATIBLE CHANGES
+
+  NEW FEATURES
+
+  IMPROVEMENTS
+
+  OPTIMIZATIONS
+
+  BUG FIXES
+
 Release 2.7.2 - UNRELEASED
 
   INCOMPATIBLE CHANGES