Revert "Upgrade Bazel to 21.0"

This reverts commit 206a76b332dbc0c1e2127e03857831225003f092.

tools/dockerfile/test/bazel/Dockerfile

@@ -46,7 +46,7 @@ RUN pip install futures==2.2.0 enum34==1.0.4 protobuf==3.5.2.post1 six==1.10.0 t
 # Bazel installation
 
 RUN apt-get update && apt-get install -y wget && apt-get clean
-RUN wget -q https://github.com/bazelbuild/bazel/releases/download/0.21.0/bazel-0.21.0-linux-x86_64 -O /usr/local/bin/bazel
+RUN wget -q https://github.com/bazelbuild/bazel/releases/download/0.17.1/bazel-0.17.1-linux-x86_64 -O /usr/local/bin/bazel
 RUN chmod 755 /usr/local/bin/bazel