Add missing __contains__() and get() methods to QuotaUsage

The QuotaUsage class was missing __contains__() and get() methods which was
causing unexpected behaviour, e.g:

    >>> usages = quotas.tenant_quota_usages(request)
    >>> 'ram' in usages
    False
    >>> usages.get('ram')
    None

Change-Id: Ib0b1ad69c4bf4effd7c10827d23bcc713bdc5ed6

openstack_dashboard/test/tests/quotas.py

@@ -119,6 +119,11 @@ class QuotaTests(test.APITestCase):
         # Compare internal structure of usages to expected.
         self.assertEqual(quota_usages.usages, expected_output)
 
+        # Make sure that the `in` operator and the `.get()` method
+        # behave as expected
+        self.assertIn('ram', quota_usages)
+        self.assertIsNotNone(quota_usages.get('ram'))
+
     @test.create_stubs({api.nova: ('server_list',
                                    'flavor_list',
                                    'tenant_quota_get',),

openstack_dashboard/usage/quotas.py

@@ -65,6 +65,9 @@ class QuotaUsage(dict):
     def __init__(self):
         self.usages = defaultdict(dict)
 
+    def __contains__(self, key):
+        return key in self.usages
+
     def __getitem__(self, key):
         return self.usages[key]
 
@@ -76,6 +79,9 @@ class QuotaUsage(dict):
     def __repr__(self):
         return repr(dict(self.usages))
 
+    def get(self, key, default=None):
+        return self.usages.get(key, default)
+
     def add_quota(self, quota):
         """Adds an internal tracking reference for the given quota."""
         if quota.limit is None or quota.limit == -1: