Add ping to robot container

Adds iputils ping package to the robot container so it can
ping vms as part of its tests.

Change-Id: I1b0e3a3f9c2e0860ceb05ffa4e04b86cc658557c
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/docker/tests/Dockerfile b/docker/tests/Dockerfile
index f191ddf..a2c4378 100644
--- a/docker/tests/Dockerfile
+++ b/docker/tests/Dockerfile
@@ -29,6 +29,7 @@
 RUN DEBIAN_FRONTEND=noninteractive apt-get --yes install \
     gettext-base \
     git=1:2.17.1-1ubuntu0.8 \
+    iputils-ping=3:20161105-1ubuntu3 \
     jq=1.5+dfsg-2 \
     libcurl4-openssl-dev=7.58.0-2ubuntu3.13 \
     libssl-dev=1.1.1-1ubuntu2.1~18.04.9