Created attachment 1398983 [details]
Ansible inventory and log file.

Description of problem:
With recent openshift-ansible, glusterfs installation fails unless "glusterfs_registry" group is also defined.

Version-Release number of the following components:

ansible 2.4.2.0
  config file = /root/.ansible.cfg
  configured module search path = [u'/root/.ansible/plugins/modules', u'/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python2.7/site-packages/ansible
  executable location = /bin/ansible
  python version = 2.7.5 (default, Aug  4 2017, 00:39:18) [GCC 4.8.5 20150623 (Red Hat 4.8.5-16)]

openshift-ansible-3.9.0-0.47.0-5-g9d319b8

How reproducible:
Always

Steps to Reproduce:
1. Install glusterfs without [glusterfs_registry] group

Actual results:
Failure summary:


  1. Hosts:    b1.lan, b2.lan, b3.lan, b8.lan
     Play:     Load kernel modules for nodes
     Task:     Ensure device mapper modules loaded
     Message:  AnsibleUndefinedVariable: Unable to look up a name or access an attribute in template string (#{{ ansible_managed }}
               dm_thin_pool
               dm_multipath
               target_core_user
               {% if inventory_hostname in groups.glusterfs or inventory_hostname in groups.glusterfs_registry %}
               dm_snapshot
               dm_mirror
               {% endif %}
               ).
               Make sure your variable name does not contain invalid characters like '-': argument of type 'StrictUndefined' is not iterable

Expected results:
Glusterfs installation works even without [glusterfs_registry] group defined.

Additional info:
Ansible run in the attachment.
Workaround: use empty [glusterfs_registry] group in the inventory.
Proposed fix (credits Matt Bruzek): 
--- glusterfs.conf.j2      2018-02-21 16:08:56.194000000 +0100
+++ glusterfs.conf.j2.fixed      2018-02-21 19:40:01.251000000 +0100
@@ -2,7 +2,7 @@
 dm_thin_pool
 dm_multipath
 target_core_user
-{% if inventory_hostname in groups.glusterfs or inventory_hostname in groups.glusterfs_registry %}
+{% if ('glusterfs' in groups and inventory_hostname in groups['glusterfs']) or ('glusterfs_registry' in groups and inventory_hostname in groups['glusterfs_registry']) %}
 dm_snapshot
 dm_mirror
 {% endif %}

With openshift-ansible-3.9.0-0.42.0-48-gdea7e61 I was able to install without any workarounds.