/etc/mysql/percona-xtradb-cluster.cnf is in percona-xtradb-cluster-server-5.7 5.7.20-29.24-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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# The Percona XtraDB Cluster 5.7 configuration file.
#
#
# * IMPORTANT: Additional settings that can override those from this file!
# The files must end with '.cnf', otherwise they'll be ignored.
# Please make any edits and changes to the appropriate sectional files
# included below.
#
!includedir /etc/mysql/conf.d/
!includedir /etc/mysql/percona-xtradb-cluster.conf.d/
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