/vars directories from 'tox' playbook and roles are merged, which
creates conflicts and wrong variables are loaded into the playbook
Prevent merge of /vars directories from roles and 'tox' playbook
by renaming variables directory of playbook to molecule-vars.
It should fix failing job tripleo-ansible-centos-7-role-addition
Change-Id: Iee6768a27db36a3a18c0728040ab9c09857d55b6