Object's fields cannot be unset with update method

[Andrey Pavlov]

There is no way to unset a current value of a field via saharaclient update methods. Providing "None" value for some parameter will have no effect because this parameter will be removed from update request by _copy_if_defined method.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/272503

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/272503
Committed: https://git.openstack.org/cgit/openstack/python-saharaclient/commit/?id=adf19185c4e9bbd3d245a15cbccb37f56b7464c5
Submitter: Jenkins
Branch: master

commit adf19185c4e9bbd3d245a15cbccb37f56b7464c5
Author: Andrey Pavlov <email address hidden>
Date: Tue Jan 26 14:39:33 2016 +0300

    Adding ability to unset fields with update calls

    Adding a sentinel object that will be used as a default
    value in update methods. This object will allow to
    distinguish whether a field should be unset or should
    not be updated.

    Change-Id: Ie6679c8f64623e8570176b8009380b08de2be6a5
    Closes-bug: #1534050