Added Trove (database) version API tests

Added a new file "test_versions.py" to verify the current
version of the Database API.  Required supporting
functions are added in a new client file "versions_client.py"
under the JSON interface.
Modified api/base.py, etc/tempest.conf.sample, clients.py and
config.py files

Partially implements blueprint: trove-tempest

Change-Id: I3dbe4e40b8b2a1ec3c69573dd40c3c8a643d73d6
Merge "Added Trove (database) version API tests"
Merge branch 'master' of https://github.com/openstack/tempest