Remove unused env variable from the fluentd dockerfiles and make their apt-get

arguments consistent.

cluster/addons/fluentd-elasticsearch/fluentd-es-image/Dockerfile

@@ -14,6 +14,9 @@ MAINTAINER Satnam Singh "satnam@google.com"
 # Ensure there are enough file descriptors for running Fluentd.
 RUN ulimit -n 65536
 
+# Disable prompts from apt.
+ENV DEBIAN_FRONTEND noninteractive
+
 # Install prerequisites.
 RUN apt-get update && \
     apt-get install -y curl && \