readers: make most JGF vertex metadata optional

Problem: as described in flux-framework#1255, JGF is too large and unwieldy,
with lots of redundant and repeated data.

To begin to cut back, make most vertex metadata optional.

resource/readers/resource_reader_jgf.cpp

@@ -341,6 +341,11 @@ int resource_reader_jgf_t::remap_aware_unpack_vtx (fetch_helper_t &f,
     return 0;
 }
 
+
+int resource_reader_jgf_t::apply_defaults (fetch_helper_t &f) {
+
+}
+
 int resource_reader_jgf_t::fill_fetcher (json_t *element,
                                          fetch_helper_t &f,
                                          json_t **paths,
@@ -354,17 +359,17 @@ int resource_reader_jgf_t::fill_fetcher (json_t *element,
         errno = EINVAL;
         m_err_msg += __FUNCTION__;
         m_err_msg += ": JGF vertex id key is not found in a node.\n";
-        goto done;
+        return -1;
     }
     if ((metadata = json_object_get (element, "metadata")) == NULL) {
         errno = EINVAL;
         m_err_msg += __FUNCTION__;
         m_err_msg += ": key (metadata) is not found in an JGF node for ";
         m_err_msg += std::string (f.vertex_id) + ".\n";
-        goto done;
+        return -1;
     }
     if ((json_unpack (metadata,
-                      "{ s:s s:s s:s s:I s:I s:I s?:i s:b s:s s:I s:o s?o }",
+                      "{ s:s s?s s?s s?I s?I s?I s?i s?b s?s s?I s?o s?o }",
                       "type",
                       &f.type,
                       "basename",
@@ -394,16 +399,16 @@ int resource_reader_jgf_t::fill_fetcher (json_t *element,
         m_err_msg += __FUNCTION__;
         m_err_msg += ": malformed metadata in an JGF node for ";
         m_err_msg += std::string (f.vertex_id) + "\n";
-        goto done;
+        return -1;
     }
     if (*properties && !json_is_object (*properties)) {
         errno = EINVAL;
         m_err_msg += __FUNCTION__;
         m_err_msg += ": key (properties) must be an object or null for ";
         m_err_msg += std::string (f.vertex_id) + ".\n";
-        goto done;
+        return -1;
     }
-    rc = 0;
+    return apply_defaults (f);
 done:
     return rc;
 }