Problem was too tight condition on ha_commit_trans to not
allow non transactional storage engines participate 2pc
in Galera case. This is required because transaction
using e.g. procedures might read mysql.proc table inside
a trasaction and these tables use at the moment Aria
storage engine that does not support 2pc.

Fixed by allowing read only transactions to storage
engines that do not support two phase commit to participate
2pc transaction. These will be committed later separately.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>