Wait for DB writes only on reads (causality checks)

On PR 392 we changed to waiting on writes for updates and inserts, but
upon further analysis we have decided that just doing it for reads
should be enough, as the order of the updates will be reconciled
automatically by the DB engine.

In this patch we change the value of `mysql_wsrep_sync_wait` to 1 to
only wait on reads.

templates/cinder/config/00-global-defaults.conf

@@ -39,10 +39,11 @@ connection = {{ .DatabaseConnection }}
 max_retries = -1
 db_max_retries = -1
 
-# Wait for writes to complete when doing a read, update, or insert
-# Relevant for multi-master deployments so that workers table works as intended
+# Wait for writes to complete when doing reads
+# Relevant for multi-master deployments so that workers table, and possibly
+# other tables, work as intended
 # https://mariadb.com/docs/server/ref/mdb/system-variables/wsrep_sync_wait/
-mysql_wsrep_sync_wait = 7
+mysql_wsrep_sync_wait = 1
 
 [os_brick]
 lock_path = /var/locks/openstack/os-brick