Add option to OVERRIDE

Providing an OVERRIDE tarball allows any and everything about the image
to be customized *before* the initial commands are run. This will be
used in the gate and other CICD systems to change package sources as
well as configure options.

In infra specfically we will be disabling signature verification on the
packages and pointing the repos to use infra repos.

Regular building is 100% unaffected by this addition.

Change-Id: I553e8e7b6fc0cb66076f1492ddd2ae4f8742db95

centos/Dockerfile

@@ -1,5 +1,9 @@
 FROM centos:7
 
+ARG OVERRIDE=override
+
+ADD $OVERRIDE /
+
 ENV OPENSTACK_VERSION=newton
 
 RUN set -x \

debian/Dockerfile

@@ -1,5 +1,9 @@
 FROM debian:jessie
 
+ARG OVERRIDE=override
+
+ADD $OVERRIDE /
+
 RUN set -x \
     # NOTE(SamYaple): backports is only used for liberasurecode-dev
     && echo "deb http://deb.debian.org/debian jessie-backports main" > /etc/apt/sources.list.d/backports.list \

ubuntu/Dockerfile

@@ -1,5 +1,9 @@
 FROM ubuntu:xenial
 
+ARG OVERRIDE=override
+
+ADD $OVERRIDE /
+
 RUN set -x \
     && apt-get update \
     && apt-get install -y --no-install-recommends \