Load dependent plugins before us if they are installed
Add OnlyOnce middleware action class

A middleware class that builds and runs the stack based on the specified
block, but only once.
Use OnlyOnce to build the middleware stack
Hook before vagrant-vbguest if it is installed
Swap our middleware running order to "normal"

Our configuration actions are now run in an isolated Runner, so
we can do all the magic before calling the next middleware.
Clean up action_hook registration

The middleware stack can now be safely hooked to many actions as it is
executed only once.
Lower the level of Action::OnlyOnce debug messages
Update changelog

[ci skip]
Add unit tests for Action::OnlyOnce
Merge pull request #30 from tmatilai/vbguest

Support vagrant-vbguest plugin