Install tempest instead of just adding it to PYTHONPATH

This commit really does a setup on tempest instead of just adding it
to the python path. The advantage of this is that entry points for
scripts like verify-tempest-config will actually be linked properly
so that they can be run directly.

Change-Id: I42a433955fde2e43ba783ccfdd08b893518358b4
Merge "Install tempest instead of just adding it to PYTHONPATH"