If the storage disks are initialized (for example for previously aborted install), heketi does not overwrite them unless using the -ff option.
it looks like that option can be managed from the ansible installer by setting the following:

openshift_storage_glusterfs_wipe: true

when I set that value if fails with the following:

TASK [openshift_storage_glusterfs : Unlabel any existing GlusterFS nodes] *****************************************************************************************************************************************
task path: /tmp/git/casl-ansible/openshift-ansible/roles/openshift_storage_glusterfs/tasks/glusterfs_deploy.yml:19
Friday 22 December 2017  18:55:58 +0000 (0:00:02.835)       0:04:49.891 ******* 
changed: [master-1.env1.casl.raffa.com] => (item=infranode-0.env1.casl.raffa.com) => {"changed": true, "failed": false, "item": "infranode-0.env1.casl.raffa.com", "results": {"cmd": "/bin/oc label node infranode-0.env1.casl.raffa.com region-", "results": {}, "returncode": 0}, "state": "absent"}
ok: [master-1.env1.casl.raffa.com] => (item=master-1.env1.casl.raffa.com) => {"changed": false, "failed": false, "item": "master-1.env1.casl.raffa.com", "state": "absent"}
ok: [master-1.env1.casl.raffa.com] => (item=master-2.env1.casl.raffa.com) => {"changed": false, "failed": false, "item": "master-2.env1.casl.raffa.com", "state": "absent"}
ok: [master-1.env1.casl.raffa.com] => (item=master-0.env1.casl.raffa.com) => {"changed": false, "failed": false, "item": "master-0.env1.casl.raffa.com", "state": "absent"}
fatal: [master-1.env1.casl.raffa.com]: FAILED! => {"failed": true, "msg": "The task includes an option with an undefined variable. The error was: 'dict object' has no attribute 'openshift'\n\nThe error appears to have been in '/tmp/git/casl-ansible/openshift-ansible/roles/openshift_storage_glusterfs/tasks/glusterfs_deploy.yml': line 19, 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: Unlabel any existing GlusterFS nodes\n  ^ here\n\nexception type: <class 'ansible.errors.AnsibleUndefinedVariable'>\nexception: 'dict object' has no attribute 'openshift'"}

I think the problem is related to the following piece of code:

- name: Unlabel any existing GlusterFS nodes
  oc_label:
    name: "{{ hostvars[item].openshift.node.nodename }}"
    kind: node
    state: absent
    labels: "{{ glusterfs_nodeselector | lib_utils_oo_dict_to_list_of_dict }}"
  with_items: "{{ groups.all }}"
  when: "'openshift' in hostvars[item] and glusterfs_wipe"

an inventory file can contain hosts that are not nodes and so this may fail:
{{ hostvars[item].openshift.node.nodename }}

I think the code should iterate only on the nodes:
with_items: "{{ groups.nodes }}"