Fixing rocksdb_hotbackup test suite failure

Summary:
After D51231 (rejecting binlog_format!=row), rocksdb_hotbackup
started failing because it used statement based binary logging.
This diff changed my.cnf in rocksdb_hotbackup sute.

Squash with: Squash with: https://reviews.facebook.net/D51231

Test Plan: mtr --suite=rocksdb_hotbackup

Reviewers: santoshb, hermanlee4, jkedgar

Reviewed By: jkedgar

Subscribers: webscalesql-eng

Differential Revision: https://reviews.facebook.net/D51597

mysql-test/suite/rocksdb_hotbackup/base.cnf

@@ -7,13 +7,14 @@ rocksdb
 default-storage-engine=rocksdb
 skip-innodb
 default-tmp-storage-engine=MyISAM
+binlog_format=row
 
 [mysqld.2]
 rocksdb
 default-storage-engine=rocksdb
 skip-innodb
 default-tmp-storage-engine=MyISAM
-
+binlog_format=row
 
 [ENV]
 MASTER_MYPORT=              @mysqld.1.port