kube/kube.libsonnet: add CertificateVolume

CertificateVolume is like SecretVolume, but for secrets generated from
Certificates.

Change-Id: I312be8e84c856221173583df478ec5317aa948c0

kube/kube.libsonnet

@@ -12,6 +12,10 @@ kube {
     Certificate(name): kube._Object("certmanager.k8s.io/v1alpha1", "Certificate", name) {
         spec: error "spec must be defined",
     },
+    # For use in PodSpec.volumes_
+    CertificateVolume(certificate): {
+        secret: { secretName: certificate.spec.secretName },
+    },
 
     CephObjectStoreUser(name): kube._Object("ceph.rook.io/v1", "CephObjectStoreUser", name) {
         local user = self,