can't upgrade xtradb cluster from 5.6 to 5.7

[Misko Lee]

Hi,All:

i am trying to upgrade xtradb cluster from 5.6 to 5.7. but its can't completed.

operator history

```sh

service mysql stop

apt-get remove percona-xtrabackup* percona-xtradb-cluster*

apt-get install percona-xtradb-cluster-57 //its has some issuse of auto start mysqld

service mysql stop

/usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib/mysql/plugin --user=mysql --wsrep-provider='none'

#a new session
mysql_upgrade

#Checking if update is needed.
#Checking server version.
#Running queries to upgrade MySQL server.
#mysql_upgrade: (non fatal) [ERROR] 1728: Cannot load from mysql.proc. The table is probably corrupted
#mysql_upgrade: (non fatal) [ERROR] 1545: Failed to open mysql.event
#mysql_upgrade: [ERROR] 1136: Column count doesn't match value count at row 1

System Version: ubuntu 16.04

source mysqld version:
mysqld Ver 5.6.28-76.1-56 for debian-linux-gnu on x86_64 (Percona XtraDB Cluster (GPL), Release rel76.1, Revision f9b078d, WSREP version 25.14, wsrep_25.14)

dist mysqld version:

mysqld Ver 5.7.20-18-57 for debian-linux-gnu on x86_64 (Percona XtraDB Cluster (GPL), Release rel18, Revision 4a4da7e, WSREP version 29.24, wsrep_29.24)