Description of problem:
Installation failed when enable openshift_storage_glusterfs_registry_block_storageclass with glusterfs_registry group

Version-Release number of the following components:
openshift-ansible-3.9.0-0.39.0.git.0.fea6997.el7
ansible-2.4.3.0-1.el7ae

How reproducible:
100%

Steps to Reproduce:
1. Install CNS with openshift_storage_glusterfs_registry_block_storageclass with glusterfs_registry group
# cat hosts
[OSEv3:children]
masters
nodes
etcd
glusterfs_registry
[OSEv3:vars]
...
openshift_hosted_registry_storage_kind=glusterfs
openshift_storage_glusterfs_registry_block_storageclass=true
openshift_storage_glusterfs_registry_block_storageclass_default=true
openshift_storageclass_default=false
...
[glusterfs_registry]
...
2.
3.

Actual results:
# ansible-playbook -i hosts -v /usr/share/ansible/openshift-ansible/playbooks/deploy_cluster.yml
...
TASK [openshift_storage_glusterfs : Generate Gluster Block StorageClass file] ***
Wednesday 07 February 2018  06:29:27 +0000 (0:00:01.619)       0:32:20.794 **** 
fatal: [ec2-xx-xx-xx-xx.compute-1.amazonaws.com]: FAILED! => {"changed": false, "msg": "AnsibleUndefinedVariable: 'glusterfs_heketi_route' is undefined"}
...

Expected results:
Installation success.

Additional info:
glusterfs_heketi_route should be determine in TASK [openshift_storage_glusterfs : Determine StorageClass heketi URL], according to install log, it was skip:
...
TASK [openshift_storage_glusterfs : Determine StorageClass heketi URL] *********
Wednesday 07 February 2018  06:27:05 +0000 (0:00:00.026)       0:29:58.288 **** 
skipping: [ec2-xx-xx-xx-xx.compute-1.amazonaws.com] => {"changed": false, "skip_reason": "Conditional result was False"}
...

Conditional of this TASK is [1] and [2], looks like default value of [2] is root cause.

[1]. https://github.com/openshift/openshift-ansible/blob/master/roles/openshift_storage_glusterfs/tasks/glusterfs_common.yml#L298
[2]. https://github.com/openshift/openshift-ansible/blob/master/roles/openshift_storage_glusterfs/tasks/glusterfs_common.yml#L313