playbook: Use correctly roles/vars

The previous usage lead to impossilibity of overriding variables defined
in role, because they were included forcibly in the tasks for
standalone-shared.

By using default Ansible apporach to role vars, they can be overriden on
playbook basis.

/reviewed-on nexedi/slapos.package!48

playbook/roles/standalone-shared/tasks/main.yml

 ---
   - include: debian9-gcc-4.9.yml
 
-  - name: fetch shared vars
-    include_vars: erp5-common.yml
-
   - name: Store public IPv4 for other scripts
     copy: content="{{ ansible_default_ipv4.address }}" dest=/tmp/playbook-public-ipv4 mode=0644