Add tests/__init__.py from Nova to fix eventlet import

As nova.test imports eventlet, nova.tests.__init__ will fail
based on eventlet being imported without 'EVENTLET_NO_GREENDNS' being
set. This isn't an issue in Nova itself as all the tests will automatically
run nova.tests.__init__ which sets this variable and imports eventlet.

Here, we introduce this code from Nova so that just as in Nova,
our unit tests first import eventlet and set this ENV variable.

Change-Id: I8e035e532a4bb79f96623641a606ed3f089b3d04
Remove unused tests/base.py

This code was introduced in the initial series of commits
but is unused. Pruning.

Change-Id: I7aecb161f275bad48d4ec1ec6f78d0cb3ba05196
Merge "Remove unused tests/base.py"