Add version constraint for coverage

Python coverage lower than 3.6 causes issues when running the whole
test suite. Some arguments are randomly mixed up with script names,
resulting in unittest failures like this:

UnexpectedMethodCallError: Unexpected method call.  unexpected:-  expected:+
- exists.__call__('/tmp/buildd/.../nova/virt/images.py') -> None
+ exists.__call__('/some/path') -> True

Change-Id: Iadb6d046e3acc75c6ed5abffcfc1abda82aeee4f

tools/test-requires

@@ -1,7 +1,7 @@
 # Packages needed for dev testing
 distribute>=0.6.24
 
-coverage
+coverage>=3.6
 discover
 feedparser
 fixtures>=0.3.12