Use '-m' for tempest_coverage.py in tox.ini

Currently you can't run tempest_coverage.py because of an import
failure when trying to import from files in tempest. This is because
the python path from tempest/tools doesn't include it's parent
directory. This commit gets around the import failure with tox by
using 'python -m' for the script.

Change-Id: Iebbbbc1fbcf7f5fb0eb5d9dbdf3901cb77b90a43
diff --git a/tox.ini b/tox.ini
index 33ca1c4..1b18586 100644
--- a/tox.ini
+++ b/tox.ini
@@ -14,9 +14,9 @@
 commands = nosetests {posargs}
 
 [testenv:coverage]
-commands = python tools/tempest_coverage.py -c start --combine
+commands = python -m tools/tempest_coverage -c start --combine
            nosetests {posargs}
-           python tools/tempest_coverage.py -c report --html
+           python -m tools/tempest_coverage -c report --html
 
 [testenv:pep8]
 deps = pep8==1.3.3