Enabling FavoredStochasticBalancer on existing cluster leaves regions unassigned

This is related to HBASE-18349.

The test that fails corresponding to this is TestFavoredStochasticLoadBalancer#testMisplacedRegions. When a region is misplaced w.r.t to the favored nodes, this balancer unassigns the region and the new RegionPlan has the source server as null leading to NPE later. This leaves the affected regions to be unassigned after the balancer run.

This is problematic especially when moving from a different balancer to the FavoredStochasticLoadBalancer because all regions would be "misplaced" in the favored balancer's run.

The fix is along the lines of HBASE-18602.