Compatibility with vagrant-aws v0.4.0

Next vagrant-aws release will remove its custom `TimedProvision` class and use only the standard `Builtin::Provision` instead: mitchellh/vagrant-aws@dd17f23.

To support both old and new versions of the plugin, we check if the `TimedProvision` class is defined before using it.

While at it, do the same check for vagrant-omnibus plugin, just in case it also changes in the future.