Allow ILM's Shrink Action to progress successfully on indices with 1 shard #33275
Description
When a user specifies the shrink action and the index it is supposed to shrink already has only 1 shard, this error crops up:
"failed_step": "shrink",
"phase": "warm",
"action_time": "1535643967009",
"rollover_alias": "thing-alias",
"phase_time": "1535643367314",
"step_info": "{\"type\":\"illegal_argument_exception\",\"reason\":\"can't shrink an index with only one shard\"}",
ILM should just noop this (or shrink should).