Use .coveragerc file

... instead of defining the omit options multiple times.

Change-Id: I14c921454dd50854da64bd1ed039d0cdcbb27f75

.coveragerc

@@ -0,0 +1,7 @@
+[run]
+branch = True
+source = ironic
+omit = ironic/tests/*
+
+[report]
+ignore_errors = True

tox.ini

@@ -62,13 +62,13 @@ commands =
 
 [testenv:cover]
 setenv = {[testenv]setenv}
-         PYTHON=coverage run --source ironic --omit='*tests*' --parallel-mode
+         PYTHON=coverage run --source ironic --parallel-mode
 commands =
   coverage erase
   stestr run --parallel-class {posargs}
   coverage combine
-  coverage report --omit='*tests*'
-  coverage html -d ./cover --omit='*tests*'
+  coverage report
+  coverage html -d ./cover
 
 [testenv:genconfig]
 sitepackages = False