image: make sure the target for "clear_cache" is valid

This commit:

- makes "both" the default target for clear_cache, as described by the
  documentation;
- makes sure an InvalidRequest exception is raised if another target is
  passed.

Change-Id: I61fccad78fc1b280395e0c590caaa2ee73586d93

openstack/image/v2/_proxy.py

@@ -101,7 +101,7 @@ class Proxy(proxy.Proxy):
         cache = self._get_resource(_cache.Cache, None)
         return cache.queue(self, image_id)
 
-    def clear_cache(self, target):
+    def clear_cache(self, target='both'):
         """Clear all images from cache, queue or both
 
         :param target: Specify which target you want to clear

openstack/image/v2/cache.py

@@ -62,8 +62,12 @@ class Cache(resource.Resource):
         :returns: The server response
         """
         headers = {}
-        if target != "both":
+        if target in ('cache', 'queue'):
             headers = {'x-image-cache-clear-target': target}
+        elif target != "both":
+            raise exceptions.InvalidRequest(
+                'Target must be "cache", "queue" or "both".'
+            )
         response = session.delete(self.base_path, headers=headers)
         exceptions.raise_from_response(response)
         return response

openstack/tests/unit/image/v2/test_cache.py

@@ -69,5 +69,22 @@ class TestCache(base.TestCase):
         session = mock.Mock()
         session.delete = mock.Mock()
 
+        sot.clear(session)
+        session.delete.assert_called_with('/cache', headers={})
+
         sot.clear(session, 'both')
         session.delete.assert_called_with('/cache', headers={})
+
+        sot.clear(session, 'cache')
+        session.delete.assert_called_with(
+            '/cache', headers={'x-image-cache-clear-target': 'cache'}
+        )
+
+        sot.clear(session, 'queue')
+        session.delete.assert_called_with(
+            '/cache', headers={'x-image-cache-clear-target': 'queue'}
+        )
+
+        self.assertRaises(
+            exceptions.InvalidRequest, sot.clear, session, 'invalid'
+        )

openstack/tests/unit/image/v2/test_proxy.py

@@ -974,3 +974,12 @@ class TestCache(TestImageProxy):
             expected_args=[self.proxy, 'both'],
         )
         mock_get_resource.assert_called_once_with(_cache.Cache, None)
+
+        mock_get_resource.reset_mock()
+        self._verify(
+            "openstack.image.v2.cache.Cache.clear",
+            self.proxy.clear_cache,
+            method_args=[],
+            expected_args=[self.proxy, 'both'],
+        )
+        mock_get_resource.assert_called_once_with(_cache.Cache, None)