Try using a jinja2 "default" filter.

inventory/group_vars/jenkins-cloud

@@ -5,8 +5,6 @@ fedmsg_certs:
 - service: shell
   owner: root
   group: root
-  ansible_fqdn: jenkins.cloud.fedoraproject.org
 - service: jenkins
   owner: root
   group: jenkins
-  ansible_fqdn: jenkins.cloud.fedoraproject.org

playbooks/groups/jenkins-cloud.yml

@@ -29,7 +29,6 @@
 
   roles:
    - base
-   - fedmsg/base
 
   tasks:
   - include: "{{ tasks }}/cloud_setup_basic.yml"
@@ -238,7 +237,8 @@
    - /srv/web/infra/ansible/vars/{{ ansible_distribution }}.yml
 
   roles:
-   - fedmsg/base
+   - name: fedmsg/base
+     fedmsg_fqdn: jenkins.cloud.fedoraproject.org
 
   handlers:
   - include: "{{ handlers }}/restart_services.yml"

roles/fedmsg/base/tasks/main.yml

@@ -76,7 +76,7 @@
 
 - name: fedmsg certs
   copy: >
-    src="{{ private }}/files/fedmsg-certs/keys/{{item['service']}}-{{ansible_fqdn}}.crt"
+    src="{{ private }}/files/fedmsg-certs/keys/{{item['service']}}-{{fedmsg_fqdn | default(ansible_fqdn)}}.crt"
     dest=/etc/pki/fedmsg/
     mode=644
     owner={{item['owner']}}
@@ -89,7 +89,7 @@
 
 - name: fedmsg keys
   copy: >
-    src="{{ private }}/files/fedmsg-certs/keys/{{item['service']}}-{{ansible_fqdn}}.key"
+    src="{{ private }}/files/fedmsg-certs/keys/{{item['service']}}-{{fedmsg_fqdn | default(ansible_fqdn)}}.key"
     dest=/etc/pki/fedmsg/
     mode=0640
     owner={{item['owner']}}