[tests] python3 dsvm tests are run with improper version

[Tom Barron]

We set the ``run`` playbook for the dummy job to indicate that we should use python 3:

   export DEVSTACK_GATE_USE_PYTHON3=True

and we do not explicitly set the PYTHON3_VERSION there.

When running the job in gate local.conf contains:

   PYTHON3_VERSION=

This causes the version to be unknown when devstack uses pip to install packages. When the version is unknown devstack's install_pip function defaults to python 2.7.

As a result manila packages get pip installed with python 2 instead of python 3 and jobs that purport to run under python 3 run with a funny mix of system packages for python3 and pip installed packages for python 2.7

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/648505

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/648505
Committed: https://git.openstack.org/cgit/openstack/manila/commit/?id=d5bd526f11e69059f1f3cdc3898c9251b04b65b8
Submitter: Zuul
Branch: master

commit d5bd526f11e69059f1f3cdc3898c9251b04b65b8
Author: Tom Barron <email address hidden>
Date: Thu Mar 28 14:46:06 2019 -0400

    [tests] Fix PYTHON3_VERSION

    The pre test hook for gate tests has a faulty test
    for whether the PYTHON3_VERSION variable is set and
    as a result puts a blank value for this variable in
    local.conf when it is not set instead of leaving it
    out of local.conf altogether. This causes jobs
    running with USE_PYTHON3=True and no explict version
    setting to actually run manila under python 2.7 because
    that is what devstack itself defaults for pip installs
    if the version is not set (or auto-detected in devstack
    when local.conf does not contain the PYTHON3_VERSION
    variable at all).

    So correct the condition in our pre-test hook so that
    it only inserts a PYTHON3_VERSION in local.conf if
    that version has been supplied.

    Closes-bug: #1822178
    Change-Id: Ie37595bde17d18770d82fb7fd6475dcfab74a543

[OpenStack Infra (hudson-openstack)]

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