Rename empty to override (future-proofing)

This will facilitate some fixes that will need to happen in the future.

Change-Id: Ie25ead70e47f75379473d42c115266ae71c0a671

centos/Dockerfile

@@ -9,7 +9,7 @@ ARG GIT_REF
 ARG GIT_REF_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG SCRIPTS_REPO=https://github.com/openstack/loci
 ARG SCRIPTS_REF=master
-ARG OVERRIDE=empty
+ARG OVERRIDE=override
 
 ADD $OVERRIDE /
 

debian/Dockerfile

@@ -9,7 +9,7 @@ ARG GIT_REF
 ARG GIT_REF_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG SCRIPTS_REPO=https://github.com/openstack/loci
 ARG SCRIPTS_REF=master
-ARG OVERRIDE=empty
+ARG OVERRIDE=override
 
 ADD $OVERRIDE /
 

ubuntu/Dockerfile

@@ -9,7 +9,7 @@ ARG GIT_REF
 ARG GIT_REF_REPO=https://git.openstack.org/openstack/${PROJECT}
 ARG SCRIPTS_REPO=https://github.com/openstack/loci
 ARG SCRIPTS_REF=master
-ARG OVERRIDE=empty
+ARG OVERRIDE=override
 
 ADD $OVERRIDE /