Track timestamps for start and end of transactions

[OpenStack Infra]

https://review.openstack.org/349901
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.

commit dd2413fd387da2a63eab4d8c2f94b52ed4680d3e
Author: Roman Prykhodchenko <email address hidden>
Date: Thu Sep 1 19:32:50 2016 +0300

    Track timestamps for start and end of transactions

    In order to enable building a history of deployment transactions
    for a custer it is necessary to keep track of start and end time
    for them. This patch adds timestamp_start and timestamp_end attributes
    to transactions, makes changes to the API serializers and introduces
    appropriate database fields to store that information

    I've added times only for 'deployment' task, which is the only
    one, needed by new UI feature. At this point there is no single
    point, where task switches to error or ready state and I'd have
    to set in in many places.

    Whem everything-is-a-graph approach will be fully implemented
    it would be very easy. So let's not add unnecessary code now.

    Co-authored by: Roman Prykhodchenko <email address hidden>
    Co-authored by: Dmitry Guryanov <email address hidden>

    DocImpact
    Closes-bug: #1593753

    Change-Id: Ib206d75a8d2215f8fd6b3f89e209c6e86ed20d0f