Fix the StopIteration error for py37

[wanghao]

In Train we will switch to use py37 for tests. There is a breaking change between Python 3.6 and 3.7 that is making it an error to raise a StopIteration exception from a function or generator（https://www.python.org/dev/peps/pep-0479/）. So we need to fix it in code.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/672667

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/672667
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=b63d025135f0755bd5b1bd311146a88289305410
Submitter: Zuul
Branch: master

commit b63d025135f0755bd5b1bd311146a88289305410
Author: wanghao <email address hidden>
Date: Thu Jul 25 16:58:28 2019 +0800

    Fit the StopIteration for py37

    In Train we will switch to use py37 for tests.
    There is a breaking change between Python 3.6 and 3.7
    that is making it an error to raise a StopIteration exception
    from a function or generator(https://www.python.org/dev/peps/pep-0479/).
    So we need to fix it in code.

    Change-Id: I003be4193944bc4176768c488f2f71b58fcbd58d
    Closes-Bug: #1837856
    Story: #2005924
    Task: #34258

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/zaqar 9.0.0.0rc1 release candidate.