Description of problem:
Did not set openshift_prometheus_image_prefix in inventory file and deployed prometheus, job failed at 'openshift_prometheus_image_prefix' is undefined", it did not take the default value "registry.access.redhat.com/openshift3/".

Error:
********************************************************************************
fatal: [host-8-242-139.host.centralci.eng.rdu2.redhat.com]: FAILED! => {
    "failed": true, 
    "msg": "The task includes an option with an undefined variable. The error was: 'openshift_prometheus_image_prefix' is undefined\n\nThe error appears to have been in '/usr/share/ansible/openshift-ansible/roles/openshift_prometheus/tasks/main.yaml': line 8, 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: Set image facts\n  ^ here\n\nexception type: <class 'ansible.errors.AnsibleUndefinedVariable'>\nexception: 'openshift_prometheus_image_prefix' is undefined"
}
	to retry, use: --limit @/usr/share/ansible/openshift-ansible/playbooks/byo/openshift-cluster/openshift-prometheus.retry
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Version-Release number of the following components:
# rpm -qa | grep openshift-ansible
openshift-ansible-lookup-plugins-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch
openshift-ansible-playbooks-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch
openshift-ansible-filter-plugins-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch
openshift-ansible-callback-plugins-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch
openshift-ansible-roles-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch
openshift-ansible-docs-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch
openshift-ansible-3.7.0-0.191.0.git.0.bc2ff60.el7.noarch

images:
prometheus-alert-buffer/images/v3.7.0-0.194.0.0
prometheus-alertmanager/images/v3.7.0-0.194.0.0
oauth-proxy/images/v3.7.0-0.194.0.0
prometheus/images/v3.7.0-0.194.0.0

How reproducible:
Always

Steps to Reproduce:
1. Deploy prometheus, inventory file see the [Additional info] part
2.
3.

Actual results:
Throw out errof 'openshift_prometheus_image_prefix' is undefined"

Expected results:
Should not be error throw out

Additional info:
[OSEv3:children]
masters
etcd


[masters]
${MASTER} openshift_public_hostname=${MASTER}

[etcd]
${ETCD} openshift_public_hostname=${ETCD}



[OSEv3:vars]
ansible_ssh_user=root
ansible_ssh_private_key_file="~/libra.pem"
deployment_type=openshift-enterprise

# prometheus
openshift_prometheus_state=present
openshift_prometheus_namespace=prometheus

openshift_prometheus_replicas=1
openshift_prometheus_node_selector={'role': 'node'}