Reindexing shrunken indices fails via the Upgrade Assistant

[elasticmachine]

Original comment by @talevy:

Kibana version: 5.6.x

Description of the problem including expected versus actual behavior:

Shrunken indices in Elasticsearch leverage internal indices for speedier allocation. The problem
with this is that there is no way to differentiate internal vs. public settings when copying over
settings to the upgraded index. For this reason, Reindexing a shrunken index fails within the Upgrade
Assistant.

It seems like the offending settings should be blacklisted from the upgraded index here:
LINK REDACTED

index.routing.allocation.initial_recovery._id should be added to this list.

Steps to reproduce:

1. create index in 5.x
2. shrink said index
3. use the reindex helper to upgrade it

Provide logs and/or server output (if relevant):

{
    "statusCode": 500,
    "error": "Internal Server Error",
    "message": "An internal server error occurred",
    "code": "ERR_CREATE_INDEX_FAILED",
    "root_cause": [{
        "type": "remote_transport_exception",
        "reason": "[ifodjsijfdo][10.0.0.1:9300][indices:admin/create]"
    }],
    "type": "illegal_argument_exception",
    "reason": "unknown setting [index.routing.allocation.initial_recovery._id] please check that any required plugins are installed, or check the breaking changes documentation for removed settings",
    "reindexedIndexName": "index-shrink-reindexed-v5"
}

[joshdover]

@talevy Sounds like in the new upgrade assistant we should blacklist this setting from being copied?

[talevy]

@joshdover this may be worth discussing, but I do not see any reason why a reindexed copy of a shrunken index should retain this setting. It is an internal setting to the resize operations, and only concerns the internals of those operations. That is why it is not a registered setting that is meant to be set on an index during index creation. So, I would imagine that this is blacklisted for that reason, yeah

[joshdover]

We removed the reindexing feature in Phase 1. This will actually be closed in a future issue.