Fix api doc in creating pre-signed url
Fix api doc in creating pre-signed url

Now the three arguments 'paths', 'methods', 'expires' in body
are all optional, but it's required in api-ref doc.

So fix it soon.

Change-Id: I61dad6daa2d030a8ae14acaf1f9bed43cf0be060

openstack/zaqar,openstack/zaqar,openstack/zaqar,openstack/zaqar