Add missing line number report, fix coverage dep

The coverage missing line number report will be helpful in the gate to
show exactly what lines were not exercised by the existing tests.

Also, when coverage was added to the test requirements file it appeared
to have broken the `inventory` tox environment because coverage was also
specified directly in the tox.ini file. Even if it does not break the
gate, being specified in both places is at best redundant.

Change-Id: Ic94313018f565e186a93c991906fbfc802a84034

tox.ini

@@ -89,13 +89,10 @@ commands =
       {toxinidir}/playbooks/*.yml"
 
 [testenv:inventory]
-deps =
-    {[testenv]deps}
-    coverage
 commands =
     coverage erase
     coverage run {toxinidir}/tests/test_inventory.py
-    coverage report --include={toxinidir}/playbooks/inventory/*
+    coverage report --show-missing --include={toxinidir}/playbooks/inventory/*
 
 [testenv:linters]
 commands =