Add model_snapshot_retention_days

This commit adds model_snapshot_retention_days
to the expected serialized json

(cherry picked from commit 43cfabb)

Author: russcam

Diff for: tests/Tests/XPack/MachineLearning/PutJob/PutJobApiTests.cs

@@ -200,7 +200,8 @@ public PutJobWithCustomRulesApiTests(MachineLearningCluster cluster, EndpointUsa
 				time_field = "@timestamp"
 			},
 			description = "Lab 1 - Simple example",
-			results_index_name = "server-metrics"
+			results_index_name = "server-metrics",
+			model_snapshot_retention_days = 1
 		};
 
 		protected override int ExpectStatusCode => 200;