oslo.db.sqlalchemy.test_base requires oslotest library that is not specified in requirements.txt

[Alexei Kornienko]

https://github.com/openstack/oslo.db/blob/0.3.0/oslo/db/sqlalchemy/test_base.py#L21

Projects that are using oslo.db in tests have to explicitly install oslotest.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/115269

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/115269
Committed: https://git.openstack.org/cgit/openstack/oslo.db/commit/?id=686005e35e2018c79cd43a62bad96d06aa5a0ea1
Submitter: Jenkins
Branch: master

commit 686005e35e2018c79cd43a62bad96d06aa5a0ea1
Author: Oleksii Chuprykov <email address hidden>
Date: Tue Aug 19 15:17:31 2014 +0300

    Added check to see whether oslotest is installed

    The projects that are using oslo.db in tests need to explicitly
    include oslotest in their test-requirements.

    Closes-bug: 1356425

    Change-Id: I0ed01e141221d19af5593857c13a656f32d0f3cf