Add stack output ansible_group_vars
Add stack output ansible_group_vars

This change adds a stack output to services/common.yaml that acts
as an interface for Ansible group variables.  Ansible vars provided
via this interface will be consumed by config-download and written
under $config-download-dir/group_vars/ where they can be accessed
by ansible commands.

Part of blueprint ansible-tasks-to-role.

Change-Id: Ib70e7dda13b4a3ed30af88906ba42c25cdc93038

dprince/tripleo-heat-templates,openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates