keystone token_flush job fails even it reports token got removed from keystone.log

Issue

The Keystone token flush job can get into a state where it will never complete because the transaction size exceeds the mysql galara transaction size - wsrep_max_ws_size (1073741824).

# grep " Total expired" /var/log/keystone/keystone.log
2016-12-08 01:33:40.530 21614 INFO keystone.token.persistence.backends.sql [-] Total expired tokens removed: 1082434
2016-12-09 09:31:25.301 14120 INFO keystone.token.persistence.backends.sql [-] Total expired tokens removed: 1084241
2016-12-11 01:35:39.082 4223 INFO keystone.token.persistence.backends.sql [-] Total expired tokens removed: 1086504
2016-12-12 01:08:16.170 32575 INFO keystone.token.persistence.backends.sql [-] Total expired tokens removed: 1087823
2016-12-13 01:22:18.121 28669 INFO keystone.token.persistence.backends.sql [-] Total expired tokens removed: 1089202

[root@overcloud-controller-0 log]# tail mysqld.log 
161208  1:33:41 [Warning] WSREP: transaction size limit (1073741824) exceeded: 1073774592
161208  1:33:41 [ERROR] WSREP: rbr write fail, data_len: 0, 2
161209  9:31:26 [Warning] WSREP: transaction size limit (1073741824) exceeded: 1073774592
161209  9:31:26 [ERROR] WSREP: rbr write fail, data_len: 0, 2
161211  1:35:39 [Warning] WSREP: transaction size limit (1073741824) exceeded: 1073774592
161211  1:35:40 [ERROR] WSREP: rbr write fail, data_len: 0, 2
161212  1:08:16 [Warning] WSREP: transaction size limit (1073741824) exceeded: 1073774592
161212  1:08:17 [ERROR] WSREP: rbr write fail, data_len: 0, 2
161213  1:22:18 [Warning] WSREP: transaction size limit (1073741824) exceeded: 1073774592
161213  1:22:19 [ERROR] WSREP: rbr write fail, data_len: 0, 2