Integrate dataset exists and create on write API

Implement the newly created dataset_exists and create_dataset from the
MongoStorageEngine.

backdrop/write/api.py

@@ -178,7 +178,7 @@ def create_collection_for_dataset(dataset_name):
     if not _allow_create_collection(request.headers.get('Authorization')):
         abort(401, 'Unauthorized: invalid or no token given.')
 
-    if db.collection_exists(dataset_name):
+    if storage.dataset_exists(dataset_name):
         abort(400, 'Collection exists with that name.')
 
     try:
@@ -191,10 +191,7 @@ def create_collection_for_dataset(dataset_name):
     if capped_size is None or not isinstance(capped_size, int):
         abort(400, 'You must specify an int capped_size of 0 or more')
 
-    if capped_size == 0:
-        db.create_uncapped_collection(dataset_name)
-    else:
-        db.create_capped_collection(dataset_name, capped_size)
+    storage.create_dataset(dataset_name, capped_size)
 
     return jsonify(status='ok', message='Created "{}"'.format(dataset_name))