Fix an issue where no test coverage report is created

Running `tox -ecover` does not generate the coverage report.
Instead the following is displayed to the user:

    No coverage report, but warnings instead:
    Coverage.py warning: Module nova-dpm was never imported.

This patch set fixes the problem.

Change-Id: Iec85cf3a6453da3630d49df29c69b55ca5075d17
Closes-Bug: #1655920

.coveragerc

@@ -1,6 +1,7 @@
 [run]
 branch = True
 source = nova_dpm
+omit = nova_dpm/tests/*
 
 [report]
 ignore_errors = True

tox.ini

@@ -24,7 +24,10 @@ commands = flake8 {posargs}
 commands = {posargs}
 
 [testenv:cover]
-commands = python setup.py test --coverage --testr-args='{posargs}'
+commands =
+  coverage erase
+  python setup.py test --coverage --coverage-package-name=nova_dpm --testr-args='{posargs}'
+  coverage report
 
 [testenv:docs]
 commands = python setup.py build_sphinx