Set the release date for 2.7.0

(cherry picked from commit d52de61544060a04a273114f55979e7e929a576c)

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

@@ -12,7 +12,7 @@ Release 2.7.1 - UNRELEASED
 
   BUG FIXES
 
-Release 2.7.0 - UNRELEASED
+Release 2.7.0 - 2015-04-20
 
   INCOMPATIBLE CHANGES
 

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

@@ -38,7 +38,7 @@ Release 2.7.1 - UNRELEASED
     HDFS-7916. 'reportBadBlocks' from datanodes to standby Node BPServiceActor
     goes for infinite loop (vinayakumarb)
 
-Release 2.7.0 - UNRELEASED
+Release 2.7.0 - 2015-04-20
 
   INCOMPATIBLE CHANGES
 

hadoop-mapreduce-project/CHANGES.txt

@@ -14,7 +14,7 @@ Release 2.7.1 - UNRELEASED
 
     MAPREDUCE-6300. Task list sort by task id broken. (Siqi Li via aajisaka)
 
-Release 2.7.0 - UNRELEASED
+Release 2.7.0 - 2015-04-20
 
   INCOMPATIBLE CHANGES
 

hadoop-yarn-project/CHANGES.txt

@@ -20,7 +20,7 @@ Release 2.7.1 - UNRELEASED
     YARN-3497. ContainerManagementProtocolProxy modifies IPC timeout conf
     without making a copy. (Jason Lowe via jianhe)
 
-Release 2.7.0 - UNRELEASED
+Release 2.7.0 - 2015-04-20
 
   INCOMPATIBLE CHANGES