Optimize AvailableSpaceBlockPlacementPolicy

for now ,we may have many nodes usage over 85%, some nodes'usage have been over 90%, if we choose three nodes as nodeA-97%,nodeB-98%,nodeC-99%, actually i don't want nodeC(99%) be choosen, for nodeC usage is really high , even random chance ,nodeC will reach 100% soon, so we can directly choose the less usage node(nodeA) if all nodes's usage are over 95%(just a example)