Prevent tests Dockerfile to use cache of osm-packages repo

Change-Id: I867590f408bb401f7bd66785645f7d185ce6861e
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>
diff --git a/docker/tests/Dockerfile b/docker/tests/Dockerfile
index 115f8fb..4874ef1 100644
--- a/docker/tests/Dockerfile
+++ b/docker/tests/Dockerfile
@@ -54,6 +54,7 @@
 RUN mv /usr/share/osm-tests/conformance-tests/ /robot-systest/
 RUN mv /usr/share/osm-tests/charm.sh /usr/sbin/charm
 
+ARG CACHE_DATE=not_a_date
 RUN git clone \
     https://osm.etsi.org/gitlab/vnf-onboarding/osm-packages.git \
     --recurse-submodules \