fix mxnet uri keys

Author: ahsan-z-khan

src/sagemaker/image_uri_config/mxnet.json

@@ -391,6 +391,8 @@
             "1.3": "1.3.0",
             "1.4": "1.4.1",
             "1.6": "1.6.0",
+            "1.7": "1.7.0",
+            "1.8": "1.8.0",
             "1.9": "1.9.0"
         },
         "versions": {