Drop useless DUMMY_ENVIRONMENT environment for cron container

Since I033c51704bfb806f6515db26b8159fae12469ba5 the DUMMY_ENVIRONMENT
environment for cron container is useless, lets drop it.

Trivial-Fix

Change-Id: Id8bf642a57b54ecafe0ee6981e044b1aba872ef7
Signed-off-by: Maksim Malchuk <maksim.malchuk@gmail.com>

ansible/roles/common/defaults/main.yml

@@ -20,14 +20,12 @@ common_services:
     privileged: True
     volumes: "{{ kolla_toolbox_default_volumes + kolla_toolbox_extra_volumes }}"
     dimensions: "{{ kolla_toolbox_dimensions }}"
-  # DUMMY_ENVIRONMENT is needed because empty environment is not supported
   cron:
     container_name: cron
     group: cron
     enabled: True
     image: "{{ cron_image_full }}"
     environment:
-      DUMMY_ENVIRONMENT: kolla_useless_env
       KOLLA_LOGROTATE_SCHEDULE: "{{ cron_logrotate_schedule }}"
     volumes: "{{ cron_default_volumes + cron_extra_volumes }}"
     dimensions: "{{ cron_dimensions }}"