Set threads based on number of cpus

This patch removes hardcode from the following options, and make
them dynamically picked based on number of cpus if not set
explicitly in the model:

 innodb_read_io_threads
 innodb_write_io_threads
 wsrep_slave_threads
 xtrabackup_parallel

Change-Id: I731197689960cfdfed6a4fd930b898172d425b1d
Related-Prod: PROD-24813
4 files changed