Sagemaker models (#3105)

* First failing test, and enough framework to run it.

* Rudimentary passing test.

* Sagemaker Notebook Support, take-1: create, describe, start, stop, delete.

* Added list_tags.

* Merged in model support from https://github.com/porthunt/moto/tree/sagemaker-support.

* Re-org'd

* Fixed up describe_model exception when no matching model.

* Segregated tests by Sagemaker entity.  Model arn check by regex..

* Python2 compabitility changes.

* Added sagemaker to list of known backends.  Corrected urls.

* Added sagemaker special case to moto.server.infer_service_region_host due to irregular url format (use of 'api' subdomain) to support server mode.

* Changes for PR 3105 comments of July 10, 2020

* PR3105 July 10, 2020, 8:55 AM EDT comment: dropped unnecessary re-addition of arn when formulating model list response.

* PR 3105 July 15, 2020 9:10 AM EDT Comment: clean-up SageMakerModelBackend.describe_models logic for finding the model in the dict.

* Optimized imports

Co-authored-by: Joseph Weitekamp <jweite@amazon.com>

moto/server.py

@@ -102,6 +102,10 @@ class DomainDispatcherApplication(object):
                 # If Newer API version, use dynamodb2
                 if dynamo_api_version > "20111205":
                     host = "dynamodb2"
+        elif service == "sagemaker":
+            host = "api.sagemaker.{region}.amazonaws.com".format(
+                service=service, region=region
+            )
         else:
             host = "{service}.{region}.amazonaws.com".format(
                 service=service, region=region