use freezer user for freezer_scheduler container

freezer scheduler runs with root privilege which is needless

Change-Id: I9345a123838dd5ff5a4f4c5d37f40ac233807104
(cherry picked from commit 91cb616612)

docker/freezer/freezer-scheduler/Dockerfile.j2

@@ -5,3 +5,5 @@ LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build
 
 {% block freezer_scheduler_footer %}{% endblock %}
 {% block footer %}{% endblock %}
+
+USER freezer