Add coverage reporting to inventory testing

Using the coverage report will show how much of the dynamic_inventory.py
script is actually exercised by tests. As of this commit, there is no
enforced coverage level, rather it is used for informational purposes
only. In the future, an enforce coverage percentage would be a good
idea.

Change-Id: Ic2ccc9ae5f306465b6990d935c82f81bb7b7956a

test-requirements.txt

@@ -1,6 +1,7 @@
 ansible>1.9,<2.0,!=1.9.6
 ansible-lint>=2.0.3,<=2.3.6
 bashate==0.5.0 # Apache-2.0
+coverage<=4.0.3 # Apache-2.0
 flake8==2.2.4
 hacking>=0.10.0,<0.11
 mccabe==0.2.1 # capped for flake8

tox.ini

@@ -89,8 +89,13 @@ commands =
       {toxinidir}/playbooks/*.yml"
 
 [testenv:inventory]
+deps =
+    {[testenv]deps}
+    coverage
 commands =
-    python {toxinidir}/tests/test_inventory.py
+    coverage erase
+    coverage run {toxinidir}/tests/test_inventory.py
+    coverage report --include={toxinidir}/playbooks/inventory/*
 
 [testenv:linters]
 commands =