Fix coverage command

... to resolve the following warning.

CoverageWarning: Module $project was never imported.

Change-Id: I099f2cf836ebe325825ded276d134eb281037427
Signed-off-by: Takashi Kajinami <kajinamit@oss.nttdata.com>

tox.ini

@@ -39,7 +39,7 @@ commands = {posargs}
 
 [testenv:cover]
 setenv =
-  PYTHON=coverage run --source $project --parallel-mode
+  PYTHON=coverage run --source debtcollector --parallel-mode
 commands =
   stestr run {posargs}
   coverage combine