barbican client doesn't use keystone correctly

[yuanying]

In magnum.common.clients, barbican client uses `keystone()._client`
directly, but `keystone().client` is better to use.

Because..

* `keystone()._client` is None if it was not loaded.
* `keystone().client` can return keystone client which has admin privilege.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/219579
Committed: https://git.openstack.org/cgit/openstack/magnum/commit/?id=77d1e32c9465d63965109a3cbefc93f6aaf54052
Submitter: Jenkins
Branch: master

commit 77d1e32c9465d63965109a3cbefc93f6aaf54052
Author: OTSUKA, Yuanying <email address hidden>
Date: Wed Sep 2 13:24:45 2015 +0900

    Fix keystone client usage in barbican client

    In magnum.common.clients, barbican client uses
    `keystone()._client` directly, but `keystone().client`
    is better. Because, keystone()._client is None is it
    is not loaded.

    This fixes it.

    Closes-Bug: #1491257
    Change-Id: I3771ebef9c2a1d3756ea4ed2f964751d1c2da43f