set cov as None if python-coverage not installed

If we didn't install python-coverage, we could not
run 'python setup.py test'.
This patch fixes this little bug.

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>

setup.py

@@ -394,6 +394,7 @@ class TestBaseCommand(Command):
             use_cov = True
         except:
             use_cov = False
+            cov = None
 
         if use_cov:
             omit = ["/usr/*", "/*/tests/*"]