Comment 7 for bug 1436093

There is still the sst part left which should touch the wsrep_sst_auth variable at runtime:

http://galeracluster.com/documentation-webpages/mysqlwsrepoptions.html
wsrep_sst_auth
Variable Scope: Global
Permitted Values Type: string
Valid Values: username:password

And also result in re-rendering of my.cnf/mysqld.cnf:

wsrep_sst_auth="sstuser:{{ sst_password }}"

~~

The source code has relevant update methods:

https://git.launchpad.net/~ubuntu-server-dev/ubuntu/+source/percona-xtradb-cluster-5.7/tree/sql/sys_vars.cc?h=ubuntu/bionic&id=debian/5.7.20-29.24-0ubuntu2.1#n6220
static Sys_var_charptr Sys_wsrep_sst_auth(
       "wsrep_sst_auth", "Authentication for SST connection",
       PREALLOCATED GLOBAL_VAR(wsrep_sst_auth), CMD_LINE(REQUIRED_ARG, OPT_WSREP_SST_AUTH),
       IN_FS_CHARSET, DEFAULT(wsrep_sst_auth), NO_MUTEX_GUARD,
       NOT_IN_BINLOG,
       ON_CHECK(wsrep_sst_auth_check),
       ON_UPDATE(wsrep_sst_auth_update));

https://git.launchpad.net/~ubuntu-server-dev/ubuntu/+source/percona-xtradb-cluster-5.7/tree/sql/wsrep_sst.cc?h=ubuntu/bionic&id=debian/5.7.20-29.24-0ubuntu2.1#n194
bool wsrep_sst_auth_update (sys_var *self, THD* thd, enum_var_type type)
{
    return sst_auth_real_set (wsrep_sst_auth);
}
https://git.launchpad.net/~ubuntu-server-dev/ubuntu/+source/percona-xtradb-cluster-5.7/tree/sql/wsrep_sst.cc?h=ubuntu/bionic&id=debian/5.7.20-29.24-0ubuntu2.1#n163
static bool sst_auth_real_set (const char* value)
// ..

And the variable is marked as dynamic in documentation:
https://www.percona.com/doc/percona-xtradb-cluster/5.5/wsrep-system-index.html#wsrep_sst_auth