Add a new tox environment to run unit tests in threading mode

It is done by disabling the eventlet patching and configuring
oslo.service backend to threading. Once oslo.service backend is
configured, it can't be reverted to eventlet. This needs to be
done before including other modules, which may include oslo.service
library.
Adds a job that run a subset of tests with eventlet patching disabled.

Change-Id: I9f8c2c5bbcf3192313cc3b309e8f2719a3bea18f
Signed-off-by: Douglas Viroel <viroel@gmail.com>

tox.ini

@@ -37,6 +37,16 @@ passenv =
   # debugging issue with fixtures and mocks.
   PYTHONOPTIMIZE
 
+[testenv:py3-threading]
+setenv =
+  OS_WATCHER_DISABLE_EVENTLET_PATCHING=true
+commands =
+  rm -f .testrepository/times.dbm
+  find . -type f -name "*.py[c|o]" -delete
+  # NOTE(dviroel): Applier still requires changes to support
+  # threading mode
+  stestr run {posargs} --exclude-regex 'applier'
+
 [testenv:pep8]
 description =
     Run style checks.