Resolve certificate references in ONC by PEM.

chromeos/test/data/network/uidata_for_openvpn_clientcert.json

 {
   "certificate_pattern": {
     "EnrollmentURI": [
       "my.enrollment.url"
     ],
-    "IssuerCARef": [
-      "def",
-      "abc"
+    "IssuerCAPEMs": [
+      "pem1",
+      "pem2"
     ],
     "Subject": {
       "CommonName": "common_name",
       "Locality": "locality",
       "Organization": "organization",
       "OrganizationalUnit": "unit"
     }
   },
   "certificate_type": "pattern",
   "onc_source": "user_policy"
 }