cephadm: add idmap.conf to nfs sample file

Otherwise deployment using this sample will fail with
"ERROR: required file missing from config-json: idmap.conf"

Fixes: https://tracker.ceph.com/issues/65155

Signed-off-by: Adam King <[email protected]>

Author: adk3798

src/cephadm/samples/nfs.json

@@ -9,6 +9,7 @@
             "",
             "%url    rados://nfs-ganesha/nfs-ns/conf-nfs.a",
             ""
-        ]
+        ],
+        "idmap.conf": ""
     }
 }