Use "multihash" for data download validation

When the Glance "multihash" is available on an image, the
glanceclient should use it instead of MD5 to validate data
downloads.  For cases in which the multihash specifies an
algorithm not available to the client, an option is added
to the image-download command that will allow fallback to
the legacy MD5 checksum verification.

Change-Id: I4ee6e5071eca08d3bbedceda2acc170e7ed21a6b
Closes-bug: #1788323

glanceclient/tests/unit/test_shell.py

@@ -965,7 +965,9 @@ class ShellTestRequests(testutils.TestCase):
             self.requests = self.useFixture(rm_fixture.Fixture())
             self.requests.get('http://example.com/v2/images/%s/file' % id,
                               headers=headers, raw=fake)
-
+            self.requests.get('http://example.com/v2/images/%s' % id,
+                              headers={'Content-type': 'application/json'},
+                              json=image_show_fixture)
             shell = openstack_shell.OpenStackImagesShell()
             argstr = ('--os-image-api-version 2 --os-auth-token faketoken '
                       '--os-image-url http://example.com '