Delete the master znode after a master crash

This is the continuation of the work done in HBASE-5844.
But we can't apply exactly the same strategy: for the region server, there is a znode per region server, while for the master & backup master there is a single znode for both.

So if we apply the same strategy as for a regionserver, we may have this scenario:
1) Master starts
2) Backup master starts
3) Master dies
4) ZK detects it
5) Backup master receives the update from ZK
6) Backup master creates the new master node and become the main master
7) Previous master script continues
8) Previous master script deletes the master node in ZK
9) => issue: we deleted the node just created by the new master

This should not happen often (usually the znode will be deleted soon enough), but it can happen.