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: Ic1e906951ba3750b372b804614dc78ec926abadd

centos/Dockerfile

@@ -8,6 +8,9 @@ ARG GIT_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG GIT_REF
 ARG GIT_REF_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG SCRIPTS=https://github.com/yaodu/common/archive/0.1.2.tar.gz
+ARG OVERRIDE=override
+
+ADD $OVERRIDE /
 
 RUN set -x \
     && yum install --setopt=tsflags=nodocs -y python curl \

debian/Dockerfile

@@ -8,6 +8,9 @@ ARG GIT_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG GIT_REF
 ARG GIT_REF_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG SCRIPTS=https://github.com/yaodu/common/archive/0.1.2.tar.gz
+ARG OVERRIDE=override
+
+ADD $OVERRIDE /
 
 RUN set -x \
     && apt-get update \

ubuntu/Dockerfile

@@ -8,6 +8,9 @@ ARG GIT_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG GIT_REF
 ARG GIT_REF_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG SCRIPTS=https://github.com/yaodu/common/archive/0.1.2.tar.gz
+ARG OVERRIDE=override
+
+ADD $OVERRIDE /
 
 RUN set -x \
     && apt-get update \