Move cold migration from conductor's manager to separate task

[Timofey Durakov]

To increase cold-migration/resize process manageability and make code clearer new task similar to live-migration should be created in conductor. While moving logic, class hierarchy for tasks should be created it allows to use similar interface for all new task classes in future.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/184495
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=83787211b3ea2734e62edc746d42ecfa72fb0386
Submitter: Jenkins
Branch: master

commit 83787211b3ea2734e62edc746d42ecfa72fb0386
Author: Timofey Durakov <email address hidden>
Date: Wed Jun 24 13:05:06 2015 +0300

    Create class hierarchy for tasks in conductor

    Conductor has 2 approaches to hande rpc api calls:
    - in place (code located in manager itself)
    - in tasks (for each operation creates new task class
    where all logic is located)
    Second approach allows to implement single responsibility
    to each method.

    In this patch, a class hierarchy for tasks is introduced.
    It allows share same public interface between tasks.
    Cold-migration/resize logic is also moved to its
    new task class. This change simplifies unit tests,
    by making them cleaner.

    Closes-Bug: #1468696

    Change-Id: I820c4a8ccc58e140f24597a52f2e01444cd6d134