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
`allocation.disable_new_allocation`, `allocation.disable_allocation`, `allocation.disable_replica_allocation`,
in favour for the enable allocation decider which has a single option `allocation.enable` wich can be set to the following values:
`none`, `new_primaries`, `primaries` and `all` (default).
Closes#4488
Clinton Gormley
Simon Willnauer
Patrick Peschlow
Konrad Feldmeier
Luca Cavanna
Igor Motov
Alexander Reelsen
Martijn van Groningen
Lee Hinman