By default server heals/creates missing directories and re-populates
`format.json`, in some scenarios when disk is down for maintainenance
it would be beneficial for users to ignore such disks rather than
mistakenly using `root` partition.
Fixes#2128
This change co-incides with another patch set which
reduces the writeQuorum requirement. With the
write quorum change it is now possible to support
6 disk configuration.
