Include requests-aws4auth in image for AWS ES support

Dockerfile

@@ -2,6 +2,7 @@ FROM alpine:3.4
 
 RUN apk --update add python py-setuptools py-pip && \
     pip install elasticsearch-curator==5.1.1 && \
+    pip install requests-aws4auth && \
     apk del py-pip && \
     rm -rf /var/cache/apk/*