you should be able to configure Airwave to send you a trigger when the master gets changed. you would need to configure it to send snmp traps and/or syslog to Airwave. i believe theres a trap that exists for this. double check the syslog guide and the mib guide from the support site.
I don't think there's a report that can tell you. it would be a great feature request though to have a "count" next to each AP on Airwave showing how many times it became master and include that in a report