fix the issue for auto-assign device name during attachment

[Terry Yao]

In current powervc driver, it does not support the volume attachment without supplying a device name. it will success in the first volume attachment and it's attached to /dev/sdb, but fails on second volume attachment as it still reserve the "/dev/sdb".
The root cause is that powervc driver sync code always set the instance root_device_name to None, although PowerVC does not use the root_device_name, but we need to set that in hosting openstack side to make it works on hosting openstack side.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/121753
Committed: https://git.openstack.org/cgit/stackforge/powervc-driver/commit/?id=e7c995a58854553587f415a374f50954634f0a18
Submitter: Jenkins
Branch: master

commit e7c995a58854553587f415a374f50954634f0a18
Author: Terry Yao <email address hidden>
Date: Mon Sep 15 13:43:23 2014 +0800

    Auto assign volume device name during attachment

    In current powervc driver code, our sync code will always set the nova
    instance root_device_name as None, this will break nova framework's
    device name auto-assign function if the device name is not provide while
    volume attachment. PowerVC itself will not set it's instance
    root_device_name from it's implemenation, for PowerVC driver, we just
    get the device name of the boot volume from the powervc side, and make
    it as the root_device_name of the nova instance.

    Closes-Bug: #1369577
    Change-Id: I7e71573e7b83ec9f8c34cfa4f0a1736b90402a90