Import the _ of i18n

[zhangjialong]

In [1] and [2],we using _ to log the error message. While, we do not import it in those files.
We should add it.

[1]https://github.com/openstack/nimble/blob/master/nimble/engine/manager.py#L109
[2]https://github.com/openstack/nimble/blob/master/nimble/engine/flows/create_instance.py#L317,

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/415406
Committed: https://git.openstack.org/cgit/openstack/nimble/commit/?id=2a44ad88ccc5ede3b8eaedae27a576ace9bb7e1d
Submitter: Jenkins
Branch: master

commit 2a44ad88ccc5ede3b8eaedae27a576ace9bb7e1d
Author: zhangjl <email address hidden>
Date: Wed Dec 28 08:36:35 2016 +0000

    Fix translations usage of _

    _ wasn't being imported here, but was being used at [1] and [2]. Import
    _ from nimble.common.i18n to fix this.

    [1]https://github.com/openstack/nimble/blob/master/nimble/engine/manager.py#L109
    [2]https://github.com/openstack/nimble/blob/master/nimble/engine/flows/create_instance.py#L317

    Close-Bug: #1652906
    Change-Id: I53f524c9a2bc3fb6a285005166375615ebe27292