MINOR: Use sha512 instead of sha2 suffix in release artifacts

As per Apache guidelines:

http://www.apache.org/dev/release-distribution#sigs-and-sums

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Damian Guy <damian.guy@gmail.com>

Closes #3844 from ijuma/fix-sha512-naming

release.py

@@ -324,7 +324,7 @@ for filename in os.listdir(artifacts_dir):
     dir, fname = os.path.split(full_path)
     cmd("Generating MD5 for " + full_path, "gpg --print-md md5 %s > %s.md5" % (fname, fname), shell=True, cwd=dir)
     cmd("Generating SHA1 for " + full_path, "gpg --print-md sha1 %s > %s.sha1" % (fname, fname), shell=True, cwd=dir)
-    cmd("Generating SHA2 for " + full_path, "gpg --print-md sha512 %s > %s.sha2" % (fname, fname), shell=True, cwd=dir)
+    cmd("Generating SHA512 for " + full_path, "gpg --print-md sha512 %s > %s.sha512" % (fname, fname), shell=True, cwd=dir)
 
 cmd("Listing artifacts to be uploaded:", "ls -R %s" % artifacts_dir)
 if not user_ok("Going to upload the artifacts in %s, listed above, to your Apache home directory. Ok (y/n)?): " % artifacts_dir):
@@ -383,11 +383,11 @@ Some suggested steps:
       wget http://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz.asc &&
       wget http://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz.md5 &&
       wget http://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz.sha1 &&
-      wget http://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz.sha2 &&
+      wget http://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/kafka-%(release_version)s-src.tgz.sha512 &&
       gpg --verify kafka-%(release_version)s-src.tgz.asc kafka-%(release_version)s-src.tgz &&
       gpg --print-md md5 kafka-%(release_version)s-src.tgz | diff - kafka-%(release_version)s-src.tgz.md5 &&
       gpg --print-md sha1 kafka-%(release_version)s-src.tgz | diff - kafka-%(release_version)s-src.tgz.sha1 &&
-      gpg --print-md sha512 kafka-%(release_version)s-src.tgz | diff - kafka-%(release_version)s-src.tgz.sha2 &&
+      gpg --print-md sha512 kafka-%(release_version)s-src.tgz | diff - kafka-%(release_version)s-src.tgz.sha512 &&
       rm kafka-%(release_version)s-src.tgz* &&
       echo "OK" || echo "Failed"
  * Validate the javadocs look ok. They are at http://home.apache.org/~%(apache_id)s/kafka-%(rc_tag)s/javadoc/