Fix tox coverage section

This commit removes the setenv parameter of testenv:cover section in
tox.ini. Recently, ''tox -e cover doesn't'' work, actually.

Change-Id: I82ac3dace47c187160dc683e0f740b0589773c0b
diff --git a/tox.ini b/tox.ini
index b3052eb..892f834 100644
--- a/tox.ini
+++ b/tox.ini
@@ -36,7 +36,6 @@
 commands = oslo-config-generator --config-file tempest/cmd/config-generator.tempest.conf
 
 [testenv:cover]
-setenv = OS_TEST_PATH=./tempest/tests
 commands = python setup.py testr --coverage --testr-arg='tempest\.tests {posargs}'
 
 [testenv:all]