* TEMPEST_FROM_SOURCE is set to true to install tempest from source.
* Added capability to use tempest rpm
* change the cirros image directory to /tmp/openstack/image
if the image gets firstly downloaded in tempest workspace,
Tempest init will fail to create tempest workspace.
* Create image_dir if not exists.
Change-Id: I349783ace314a3575e93e6c853a7756f0f9b85ac