Use relative path in data_files in setup.cfg

The absolute path causes failures when installing in a virtual
environment, not having root privileges.

This change solves the installation in a virtual environment,
but it does not fix yet the tempest init workflow when installing
into a virtual environment.

Partially-fixes bug #1480389

Change-Id: Iddefb8db77ffe35a84fb35b6f9e7e823de4ab45f

setup.cfg

@@ -22,7 +22,7 @@ classifier =
 packages =
     tempest
 data_files =
-    /etc/tempest = etc/*
+    etc/tempest = etc/*
 
 [entry_points]
 console_scripts =