The following configuration options added:

    galera_innodb_read_io_threads: 8
    galera_innodb_write_io_threads: 8
    galera_wsrep_slave_threads: 8
    galera_xtrabackup_parallel: 4
    galera_error_log_enabled: false

Change-Id: Id1185c3a38669c37dc88806b4e9955522629ce1c
Related-Bug: https://mirantis.jira.com/browse/PROD-16528

Galera master installation hangs on installation
mysql-server-wsrep-5.6. It seems that log_error is not working,
because of next message: "mysqld_safe Can't log to error log and
syslog at the same time. Remove all --log-error configuration
options for --syslog to take effect." Using "syslog = yes" instead
of log_error makes it works without any other changes.

Change-Id: I39a8a87ed048557c19f3843b97c75469385e4978
Closes-bug: PROD-9065