provisioners/ansible: Fix Group Management

- Support arbitrary depth of "groups of groups of ... groups"
- Skip ':vars' suffix, but allow group names with ':' (yes, Ansible
  accepts this character)
- Like for groups of machines, groups of groups can result "empty", but
  it is not an issue for Ansible. Recursive filter on the group tree is
  a bit hard to implement, and don't brind real added value at Vagrant
  level.
provisioners/ansible: Change --limit behaviours

- The implicit default limit is always set
- ansible.limit as an empty string won't disable the default limit, but
  will be passed as "--limit=" argument and ansible-playbook will return
  an error (provided host list is empty)