Rework logic for setting threads in my.cnf

This patch updates logic for setting the following parameters
  * innodb_read_io_threads
  * innodb_write_io_threads
  * wsrep_slave_threads

When they are set to 0 (default value) number of threads will be
calcullated dynamically based on number of CPU cores but not higher
than 12.

Default number for paralell percona backups is decreased to 1.

Change-Id: I8510a2dc90f6bcce8e2f7e7ff2fd93853fdf665c
Related-Prod: PROD-24979
diff --git a/metadata/service/master/single.yml b/metadata/service/master/single.yml
index f71fc6e..7ac04d9 100644
--- a/metadata/service/master/single.yml
+++ b/metadata/service/master/single.yml
@@ -11,6 +11,10 @@
     galera_max_connections: 20000
     galera_innodb_buffer_pool_size: 3138M
     galera_error_log_enabled: true
+    galera_innodb_read_io_threads: 0
+    galera_innodb_write_io_threads: 0
+    galera_wsrep_slave_threads: 0
+    galera_xtrabackup_parallel: 0
     galera_error_log_path: /var/log/mysql/error.log
   galera:
     master:
@@ -28,6 +32,10 @@
         port: 4567
       max_connections: ${_param:galera_max_connections}
       innodb_buffer_pool_size: ${_param:galera_innodb_buffer_pool_size}
+      innodb_read_io_threads: ${_param:galera_innodb_read_io_threads}
+      innodb_write_io_threads: ${_param:galera_innodb_write_io_threads}
+      wsrep_slave_threads: ${_param:galera_wsrep_slave_threads}
+      xtrabackup_parallel: ${_param:galera_xtrabackup_parallel}
       error_log_enabled: ${_param:galera_error_log_enabled}
       error_log_path: ${_param:galera_error_log_path}
   mysql: