[Fixed] Specify default layout grid (#1296)

* [Fixed] Specify default layout grid

Previously users would see blank values for grid size
even though a default was applied.  Users will now
see default values for grid size which should prevent
them from accidentally changing grid size.

Fixes https://github.com/nasa/openmct/issues/1285

* [Fixed] Don't hardcode grid size

Remove hardcoded grid size per comments.

https://github.com/nasa/openmct/pull/1296#issuecomment-259231499

* [Fixed] read layout before initial refresh

Read the layoutGrid sizes from scope (when watch is registered)
before refreshing elements, to ensure that layoutGrid is set
properly.

https://github.com/nasa/openmct/pull/1296#issuecomment-259231499

* [Spec] Update spec to match

Update spec to trigger watch on initialization to ensure
layoutGrid is set.

https://github.com/nasa/openmct/pull/1296

platform/features/fixed/bundle.js

@@ -204,6 +204,7 @@ define([
                         }
                     ],
                     "model": {
+                        "layoutGrid": [64, 16],
                         "composition": []
                     },
                     "properties": [