Stop gating on unit test coverage

It's nice to keep coverage high, but with little active development
it starts merely hindering refactorings, causing us to gradually lower
the expected minimum anyway.

Change-Id: I4bec5dbb224c59048c24cf2189a80014d63bfa36

tox.ini

@@ -48,7 +48,7 @@ commands =
     coverage erase
     stestr run {posargs}
     coverage combine
-    coverage report -m --omit='*test*' --fail-under 90
+    coverage report -m --omit='*test*'
     coverage html -d ./cover --omit='*test*'
 
 [testenv:pep8]