Load openstack_version for plugin_version

[BobbyRyterski]

Saw this on a fresh bundle install:

E, [2015-09-16T13:33:21.697624 #98976] ERROR -- Kitchen: ------Exception-------
E, [2015-09-16T13:33:21.697750 #98976] ERROR -- Kitchen: Class: Kitchen::ClientError
E, [2015-09-16T13:33:21.697780 #98976] ERROR -- Kitchen: Message: Could not load the 'openstack' driver from the load path. Please ensure that your driver is installed as a gem or included in your Gemfile if using Bundler.
E, [2015-09-16T13:33:21.697806 #98976] ERROR -- Kitchen: ---Nested Exception---
E, [2015-09-16T13:33:21.697837 #98976] ERROR -- Kitchen: Class: NameError
E, [2015-09-16T13:33:21.697858 #98976] ERROR -- Kitchen: Message: uninitialized constant Kitchen::Driver::OPENSTACK_VERSION

This PR loads openstack_version to define the constant.

[david-crowder]

Got the same error and this fork resolved it.

[jjasghar]

Thanks for this, just released another dev release gem.