Allow docker engine to be pulled from s3 bucket

Use aws cli to pull docker engine binaries from s3 bucket.  Conditional and
happens if DOCKER_BIN_URL begins with 's3://'

Example usage:
```
  export AWS_ACCESS_KEY_ID=key
  export AWS_SECRET_ACCESS_KEY=secret
  make DOCKER_BIN_URL=s3://path-to-engine-binaries
```
Signed-off-by: Robb Kistler <robb.kistler@docker.com>

Signed-off-by: Robb Kistler <robb.kistler@docker.com>

tools/aws-cli/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:3.5
+
+RUN \
+	mkdir -p /aws && \
+	apk -Uuv add groff less python py-pip && \
+	pip install awscli && \
+	apk --purge -v del py-pip && \
+	rm /var/cache/apk/*
+
+WORKDIR /aws
+ENTRYPOINT ["aws"]