Remove unnecessary dict.keys() method calls (api)

Since iter(dict) is equivalent to iter(dict.keys()), it is unnecessary
to call the keys() method of a dict, the dictionary itself is enough
to be referenced. The shorter form is also considered to be more
Pythonic.

This patch removes the unnecessary dict.keys() method calls in api.
This is a part of a larger patch series that removes dict.keys()
method calls.

TrivialFix

Change-Id: I29000f1f05b90d70109fa01393e97e1ebf450c63

watcher/api/controllers/v1/utils.py

@@ -57,7 +57,7 @@ def validate_sort_dir(sort_dir):
 def validate_search_filters(filters, allowed_fields):
     # Very lightweight validation for now
     # todo: improve this (e.g. https://www.parse.com/docs/rest/guide/#queries)
-    for filter_name in filters.keys():
+    for filter_name in filters:
         if filter_name not in allowed_fields:
             raise wsme.exc.ClientSideError(
                 _("Invalid filter: %s") % filter_name)