In that case, the master discovery mechanism needs to be fool proof if the AP reboots.
You can use one of the following methods:
1. Master discovery can be set to use ADP (Aruba discovery protocol) .(provided ADP broadcast or multicast packets can reach the controllers)
2. Use DNS based mechanism for master discovery.
We can add a DNS record for aruba-master & add IP addresses of both the controllers.
So, when AP will discover the master via DNS, it will be presented with two IP's. It will try to reach the first IP & if that is unreahcable it will try for the 2nd one.