change: install SageMaker Python SDK into Python 3 images (#295)

Author: laurenyu

docker/1.15.2/py3/Dockerfile.cpu

@@ -100,6 +100,7 @@ RUN pip install --no-cache-dir -U \
     keras==2.3.1 \
     requests==2.22.0 \
     smdebug==0.5.0.post0 \
+    sagemaker==1.50.17 \
     sagemaker-experiments==0.1.3 \
     mpi4py==3.0.2 \
     "cryptography>=2.3" \

docker/1.15.2/py3/Dockerfile.gpu

@@ -137,6 +137,7 @@ RUN pip install --no-cache-dir -U \
     requests==2.22.0 \
     keras==2.3.1 \
     smdebug==0.5.0.post0 \
+    sagemaker==1.50.17 \
     sagemaker-experiments==0.1.3 \
     mpi4py==3.0.2 \
     "cryptography>=2.3" \