[ROUTING] Add rebalance enabled allocation decider

This commit adds the ability to enable / disable relocations
on an entire cluster or on individual indices for either:

 * `primaries` - only primaries can rebalance
 * `replica` - only replicas can rebalance
 * `all` - everything can rebalance (default)
 * `none` - all rebalances are disabled

similar to the allocation enable / disable functionality.

Relates to #7288
[TESTS] Change test node watermarks for DiskThresholdDecider

Ensures that we can still run tests if the machine running them does not
have enough free disk space to be below the high watermark
[STORE] Rethrow FNF exception if commit-point is provided

If the store tries to read metadata from a directory it might be
empty or have no valid index inside. This case we should just
return an empty map. Yet if a commit point is present this exception
should be bubbled up.