Description of problem:
infra role expect variable hosts but in cluster are already added some hosts
or
user can setup only parts in which host isn't required
- create empty clusters, add users...

Version-Release number of selected component (if applicable):
ovirt-ansible-infra-1.1.2-1.el7ev.noarch
ansible-2.4.1.0-1.el7ae.noarch


Expected results:
variable hosts shouldn't be mandatory, if user wants to add a storage domain without a host in data center, it should fail with appropriate error message 

Additional info:
fatal: [localhost]: FAILED! => {
    "failed": true, 
    "msg": "The task includes an option with an undefined variable. The error was: 'hosts' is undefined\n\nThe error appears to have been in '/usr/share/ansible/roles/oVirt.infra/roles/oVirt.hosts/tasks/main.yml': line 2, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n---\n- name: Get hosts\n  ^ here\n\nexception type: <class 'ansible.errors.AnsibleUndefinedVariable'>\nexception: 'hosts' is undefined"
}