Change dbsubnetgroup.DBSubnetGroup from Listing<String|Mapping> to Listing<String|Mapping>|Mapping

[henriqueleite42]

Currently, it doesn't support this kind of use:

Resources {
	["SubnetGroup"] = new dbsubnetgroup.DBSubnetGroup {
		// ...
		SubnetIds = cfn.Ref("SubnetsIds")
		// ...
	}
}

It forces you to receive multiple subnet parameters to be passed directly in a listing, like this:

Resources {
	["SubnetGroup"] = new dbsubnetgroup.DBSubnetGroup {
		// ...
		SubnetIds = new Listing {
			cfn.Ref("SubnetId1")
			cfn.Ref("SubnetId2")
			cfn.Ref("SubnetId3")
		}
		// ...
	}
}

The idea is to fix it and allow using refs directly

[ericzbeard]

Seems like we need to do this for all array types.