This removes the need for the validation workaround for Docker, because
box_updated is once again available in that context.
We don't technically need the SyncedFolder priorities back on the Ruby
side, but wiring them through for symmetry.
Depends on https://github.com/hashicorp/vagrant-plugin-sdk/pull/183
* Populates ComponentOptions into plugin structs
* Maps options for legacy Provider Plugins into PluginOptions
* Demos use of PluginOptions in a stub provider
* Honors plugin priority and defaultable settings
Updated to properly handle sub managers. Configurators
from parent managers properly applied to instances. Instances
are cached within requesting manager. Parent loading is also
handled in calling manager to properly support parent caching.
Closers implementation replaced with cleanup.
