Finish stestr migration

Nova switched to using stestr for all it's in tree testing except for
the coverage job. This commit updates that job so it also uses stestr
and cleans up the last bits of testr setup left in tree. As part of
this change this moves all the one off per job configuration into the
tox job definitions instead of hiding it in either the .stestr.conf or
in a wrapper script.

Change-Id: I36e0e791d485b44641a1ed8770eca8e95aca19be

.stestr.conf

@@ -1,4 +1,3 @@
 [DEFAULT]
-test_path=${OS_TEST_PATH:-./nova/tests}
+test_path=./nova/tests/unit
 top_dir=./
-group_regex=nova\.tests\.functional\.api\.openstack\.placement\.test_placement_api(?:\.|_)([^_]+)