Allocation: Change high/low watermark defaults for disk based allocation decider

The current values for

`cluster.routing.allocation.disk.watermark.low
cluster.routing.allocation.disk.watermark.high`

are too low -- 70% and 85%.  On a 4 TB disk this means we can leave 1.2 TB unallocated.  

We should decide on and implement new, higher defaults.   I'll propose low=85% and high=90%.

We should also update the docs as part of this work as they reference the defaults.
