Increase threads for Galera slaves

Set galera wsrep_threads to at least
4 or 2*physical_cpus but not more than
12.

Low number of slave threads leads to
primary controller node failing to
send state transfer and thus to
deployment failure.

Change-Id: Ic2cfe8a50ec61a2f64562f0db64fe7a74b5d04a5
Closes-Bug: 1350245
Merge "Increase threads for Galera slaves"