Add 'explain' option for cluster reroute command to get details feedback

[dakrone]

We should add an explain parameter so people can get detailed feedback about why a shard can or cannot be allocated to a node. Something like this:

For these commands:

curl -XPOST 'localhost:9200/_cluster/reroute?explain&pretty' -d '{
  "commands" : [
    {
      "cancel" : {
        "index" : "decide", "shard" : 0, "node": "IvpoKRdtRiGrQ_WKtt4_4w"
      }
    },
    {
      "move" : {
        "index" : "decide", "shard" : 0,
        "from_node" : "IvpoKRdtRiGrQ_WKtt4_4w", "to_node" : "IvpoKRdtRiGrQ_WKtt4_4w"
      }
    }
  ]
}'

This result:

{
  "explanations" : [ {
    "command" : "cancel",
    "parameters" : {
      "index" : "[decide]",
      "shard" : 0,
      "node" : "IvpoKRdtRiGrQ_WKtt4_4w",
      "allow_primary" : false
    },
    "decisions" : [ {
      "decider" : "CancelAllocationCommand",
      "decision" : "NO",
      "explanation" : "can't cancel [decide][0] on node [Wysper][IvpoKRdtRiGrQ_WKtt4_4w][Xanadu.local][inet[/172.16.1.8:9300]], shard is primary and started"
    } ]
  }, {
    "command" : "move",
    "parameters" : {
      "index" : "[decide]",
      "shard" : 0,
      "from_node" : "IvpoKRdtRiGrQ_WKtt4_4w",
      "to_node" : "IvpoKRdtRiGrQ_WKtt4_4w"
    },
    "decisions" : [ {
      "decider" : "SameShard",
      "decision" : "NO",
      "explanation" : "shard cannot be allocated on same node [IvpoKRdtRiGrQ_WKtt4_4w] it already exists on"
    }, {
      "decider" : "Filter",
      "decision" : "YES",
      "explanation" : "node passes include/exclude/require filters"
    }, {
      "decider" : "ReplicaAfterPrimaryActive",
      "decision" : "YES",
      "explanation" : "shard is primary"
    }, {
      "decider" : "Throttling",
      "decision" : "YES",
      "explanation" : "below shard recovery limit of [2]"
    }, {
      "decider" : "Enable",
      "decision" : "YES",
      "explanation" : "allocation disabling is ignored"
    }, {
      "decider" : "Disable",
      "decision" : "YES",
      "explanation" : "allocation disabling is ignored"
    }, {
      "decider" : "Awareness",
      "decision" : "YES",
      "explanation" : "no allocation awareness enabled"
    }, {
      "decider" : "ShardsLimit",
      "decision" : "YES",
      "explanation" : "total shard limit disabled: [-1] <= 0"
    }, {
      "decider" : "NodeVersion",
      "decision" : "YES",
      "explanation" : "target node version [2.0.0-SNAPSHOT] is same or newer than source node version [2.0.0-SNAPSHOT]"
    }, {
      "decider" : "DiskThreshold",
      "decision" : "YES",
      "explanation" : "disk threshold decider disabled"
    }, {
      "decider" : "SnapshotInProgress",
      "decision" : "YES",
      "explanation" : "no snapshots are currently running"
    } ]
  } ]
}

[dakrone]

Moved to #2483 (closing this as duplicate)