CS-14826, CS-14996: fix the md5 checksum

Reviewed-by: sheng.yang@citrix.com

core/src/com/cloud/storage/template/DownloadManagerImpl.java

@@ -293,7 +293,7 @@ public class DownloadManagerImpl implements DownloadManager {
             }       
             byte[] md5sum = digest.digest();
             BigInteger bigInt = new BigInteger(1, md5sum);
-            checksum = bigInt.toString(16);
+            checksum = String.format("%032x",bigInt.toString(16));
             return checksum;
         }catch(IOException e) {
         	return null;