Set libvirt exporter image in default role

drive-by: fix vencrypt issuer config
diff --git a/roles/libvirt/vars/main.yml b/roles/libvirt/vars/main.yml
index 37e2ce8..ed0d29f 100644
--- a/roles/libvirt/vars/main.yml
+++ b/roles/libvirt/vars/main.yml
@@ -31,5 +31,6 @@
       default_tls_x509_verify: "1"
       vnc_tls: "1"
     vencrypt:
-      issuer: Issuer
-      name: libvirt-vnc-ca
+      issuer:
+        kind: Issuer
+        name: libvirt-vnc-ca