Make the tests actually run

Firstly, add a missing oslo/version/__init__.py.

Secondly, move tests from oslo/version/tests/ to tests/

Having the tests under oslo.version and using oslo.sphinx in the same
virtual env doesn't seem to work. The issue appears to be that because
the tests don't get installed under the namespace package in the venv's
site-packages, testr can't find them.

Personally, I'm a fan of keeping tests in a different directory from the
runtime code and this is what works for oslo.config and oslo.messaging.

Thirdly, avoid using 'setup.py develop' with tox.

There seems to be an issue with the 'setup.py develop' approach caused
by the fact that oslo is a namespace package and we install oslo.sphinx
into this namespace in the virtualenv. This results in the oslo.version
package not being found when it's not properly installed into the
virtualenv.

These really should be three separate commits, but any one of them alone
causes testr to stop pretending everything is fine so they all need to
be merged together.

Closes-Bug: #1242614
Change-Id: I5140fa62dc8bdd10a58bd2847b27514a926e3c19