tempest/run: source venv only if it is present

Change-Id: Ibb15c751861b9e9e34b8cb970c9b0956a76b83bf
tempest/setup: fix tempest rpm installation

Change-Id: I811f2098a26e20dba08a4519fd00a273a47d1582
tempest: renames rpm to openstack-tempest

Change-Id: Icc75aa180c5ea4c5bcc9e91dedd21d05ebe8fc44
tempest: adapt to new tempest rpm

Change-Id: I048707d7be50e55e2ce251d85389a27e172e1eab