Improvements on Docker image for OVS

Enhance the OVS image by removing the unnecessary OVS_VERSION
environment variable, cleaning up folders to reduce the image size and
adding the Debian package tini to maintain compatibility with upstream

Test Plan:
PASS: Build stx-ovs debian image
PASS: Manually upload stx-ovs built image to a Standard system,
      use helm-override to change the openvswitch_db_server and
      openvswitch_vswitchd, container images and
      reapply stx-openstack.
PASS: Check if the openstack pods start successfully

Story: 2011385
Task: 51857
Change-Id: I2a17ea914737f795ea0c5992b75d79844fae102b
Signed-off-by: Thiago Miranda <tmarques@windriver.com>

networking/openvswitch/debian/docker/Dockerfile

@@ -5,8 +5,6 @@ ARG BASE
 
 FROM ${BASE}
 
-ENV OVS_VERSION=2.15.0
-
 RUN apt-get -y update \
  && apt-get -y install \
     openvswitch-common \
@@ -16,4 +14,15 @@ RUN apt-get -y update \
     openvswitch-pki \
     openvswitch-switch \
     openvswitch-testcontroller \
-    openvswitch-vtep
+    openvswitch-vtep \
+    tini
+
+# Ensure that tini is located at the root /tini
+RUN ln -s /usr/bin/tini /tini
+
+RUN rm -rf \
+    /var/cache/debconf/* \
+    /var/lib/apt/lists/* \
+    /var/log/* \
+    /tmp/* \
+    /var/tmp/*