Add details on how to run Tempest manually.
Add details on how to run Tempest manually.

Adds details on how to actiavte the virtual environment for
running Tempest manually.

Change-Id: If1900b00305499d91771ed88309935299e598548

stackforge/refstack-client,stackforge/refstack-client,openstack/refstack-client,openstack/refstack-client