Allow wsrep_dirty_reads to be set globally

I would like to request that wsrep_dirty_reads be elevated to a global variable so it can be set in my.cnf. A lot of people have complex application configurations that would make appending this before each query a difficult task. MySQL has made this variable a global one as of 5.6.26-25.12. I think it would be a good idea for MariaDB to do it as well. https://www.percona.com/doc/percona-xtradb-cluster/5.6/wsrep-system-index.html#wsrep_dirty_reads