Allow passing the if_exists or if_not_exists modifiers for creating a… #1045

Job	Run time
check_format	1m 38s
specs (shard.yml, 12, 1.6.2, false)	4m 7s
specs (shard.yml, 12, latest, false)	4m 12s
specs (shard.yml, 13, 1.6.2, false)	4m 3s
specs (shard.yml, 13, latest, false)	3m 43s
specs (shard.yml, 14, 1.6.2, false)	4m 1s
specs (shard.yml, 14, latest, false)	3m 49s
specs (shard.edge.yml, latest, 14, true)	3m 56s
specs (shard.yml, nightly, 14, true)	3m 46s
	33m 15s

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