mysql-community-server5.7.18 conflicts file /etc/my.cnf with Percona-Server-shared-51-5.1.73-rel14.12.625.rhel6.x86_64
try:


Download mysql-community-libs-compat from mysql (https://dev.mysql.com/downloads/mysql/)

rpm -i mysql-community-libs-compat-5.7.17-1.el7.x86_64.rpm

Install perl-DBD-MySQL 

yum install perl-DBD-MySQL

Install percona-xtrabackup

yum install percona-xtrabackup