wsrep_on should not be changeable at runtime

The wsrep_on variable can be changed at runtime with set global.
In a galera cluster this is very risky because if even a single statement
lands on the node while this variable is OFF, the entire cluster is compromised
as the change will not reach the other nodes.

(This actually happened at a customer of ours)

Please, make this variable only settable from config file at startup,
as there are no useful use cases to have this dynamic.