This tests that GroupBasedBalancer will use data in zk to do balancing during master startup.
This does not test retain assignment.
The tests brings up 3 RS, creates a new RS group 'my_group', moves 1 RS to 'my_group', assigns
'hbase:rsgroup' to 'my_group', and kill the only server in that group so that 'hbase:rsgroup'
table isn't available. It then kills the active master and waits for backup master to come
online. In new master, RSGroupInfoManagerImpl gets the data from zk and waits for the expected
assignment with a timeout.