Add per-cache-pool default replication num configuration

For now, hdfs cacheadmin can not set a default replication num for cached directive in the same cachepool. Each cache directive added in the same cache pool should set their own replication num individually.

Consider this situation, we add daily hive table into cache pool "hive" .Each time i should set the same replication num for every table directive in the same cache pool.

I think we should enable setting a default replication num for a cachepool that every cache directive in the pool can inherit replication configuration from the pool. Also cache directive can override replication configuration explicitly by calling "add & modify directive -replication" command from cli.