New settings for index-shard assignment

The following settings are needed for handling index-shard assignment.  This is for supporting index fast rebalancing:

1) Limit on the disk size per shard (250GB)

2) Threshold on shard disk size for shard to be eligible for new instance (50% or 125GB)

3) Total size of flush buffer as a percentage of plasma quota (4%)

4) Maximum number of shards per tenant (2000)

5) Flush Buffer Size for capacity calculation (1MB)

6) Number of shards per tenant as ratio of number of shards per node (5x)