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
The code running in the help subcommand relies on Command plugins being
returned from the manager with their options hash. Stubbing in a blank
hash prevents the command from crashing
What we really need is for CommandOptions to be modeled and passed
through from Go to Ruby so :primary subcommands can be displayed, but
that can be a follow-on task from this quick fix.
This uses the new Push plugin support added to the plugin SDK in https://github.com/hashicorp/vagrant-plugin-sdk/pull/106 to make the following changes:
* The plugin manager on the Go side now registers push plugins
* The the _remote_ plugin manager on the Ruby side now calls over to
the go side to get push plugins
* All the wiring is hooked up such that when a push plugin is replaced
with its remote GRPC-client-wielding equivalent, the messages are
ferried around.
