Make use of REST client from tempest project

1. run_tests.sh: Add tempest source tree to PYTHONPATH before running tests.

2. Adjust tests to use tempest client.

3. Enable authentication in devstack solum service.

For this to work, tempest must be installed. Patch [1] is submitted for the same.

[1] https://review.openstack.org/64062

Change-Id: Idaaf637c066444e02cd35da1f5dba717f0d61c6b
Merge "Make use of REST client from tempest project"