refactor(image): fix base ubuntu image for cstor containers (#156)

Using ubuntu:16.04 as base image results in flaky builds
if the mutable tag (16.04) is updated with a newer image.

Saved the ubuntu image that was working as openebs/cstor-ubuntu:xenial-20181005

Signed-off-by: kmova <kiran.mova@openebs.io>

docker/Dockerfile.base

@@ -3,7 +3,7 @@
 # libraries.
 #
 
-FROM ubuntu:16.04
+FROM openebs/cstor-ubuntu:xenial-20181005
 
 RUN apt-get clean && \
     rm -rf /var/lib/apt/lists/* && \