This command makes fuel2 client generate an ansible inventory file
based on the list of cluster nodes. Nodes are grouped by roles, if a
particular node has more than one role, it will appear in the
inventory file several times.
More details on the ansible inventory file format:
http://docs.ansible.com/ansible/intro_inventory.html
This could be useful when managing Fuel deployed clusters by the
means of ansible.
Change-Id: I2bcb5756ccbeae96f3614e1bda3caf7c8289973f