Optional check for test-requirements.txt

[Aleksey Zvyagintsev]

I'm trying to build 'gold' standard rally docker image, with constrained requirements.
Like example, im installing wide plugin :
```
RUN git clone hhttps://github.com/openstack/tempest-horizon tempest-horizon-plugin ; \
    pushd tempest-horizon-plugin ; git checkout $HORIZON_TAG; \
    pip install --constraint /var/lib/openstack_requirements/upper-constraints.txt -r requirements.txt ; \
    pip install . ; popd;
```

In general - everything works. Issue appear on extension import time:

rally verify create-verifier --type tempest \
    --name tempest-verifier \
    --source /var/lib/tempest \
    --version $TEMPEST_TAG \
    --system-wide

rally verify add-verifier-ext --source /var/lib/tempest-horizon-plugin --version ${HORIZON_TAG}

The issue, that some plugins
https://github.com/openstack/designate-tempest-plugin/blob/master/test-requirements.txt
https://github.com/openstack/tempest-horizon/blob/master/test-requirements.txt
contain test-requirements.txt

And via current rally code, rally will check for those not needed actually requirements:
https://github.com/openstack/rally/blob/stable/0.9/rally/plugins/openstack/verification/tempest/manager.py#L146-L149

I would like to propose remove those check, or at least make it optional.