Update the magnum client to send the latest version.
Update the magnum client to send the latest version.

In Magnum client, Magnum calls should include the OpenStack-API-Version
header with the value of 'latest' as default, so that the latest
version of Magnum API is run.

Change-Id: I324a88d5e70ea9d68e7caf59061f5ad0807d37aa

openstack/python-magnumclient