tox.ini: Fix cover by giving the source directory explicitly

By default, testr assumes that the name of distribution
([metadata]name in setup.cfg) matches the name of the source directory.
However, they are different for this project.  Fix the problem by
giving the source directory explicitly via an option provided by pbr.

Closes-Bug: #1502002
Change-Id: Ib17d53230e5a36a028438c0f2919f07c6013bfe2

tox.ini

@@ -22,7 +22,7 @@ commands = flake8
 commands = {posargs}
 
 [testenv:cover]
-commands = python setup.py testr --coverage --testr-args='{posargs}'
+commands = python setup.py testr --coverage --coverage-package-name=neutron_taas --testr-args='{posargs}'
 
 [testenv:docs]
 commands = python setup.py build_sphinx