Fix some bugs in provider options handling

- Defaultable needs to default to true when it's not specified
- We need to allow a non-defaultable provider to be selected if it shows
  up in the config

internal/core/project.go

@@ -161,8 +161,21 @@ func (p *Project) DefaultProvider(opts *core.DefaultProviderOptions) (string, er
 		}
 
 		plugOpts := plug.Options.(*component.ProviderOptions)
+		logger.Debug("got provider options", "options", fmt.Sprintf("%#v", plugOpts))
+
+		// Skip providers that can't be defaulted, unless they're in our
+		// config, in which case someone made our decision for us.
 		if !plugOpts.Defaultable {
-			logger.Debug("skipping non-defaultable provider", "provider", pp.Name)
+			inConfig := false
+			for _, cp := range configProviders {
+				if cp == pp.Name {
+					inConfig = true
+				}
+			}
+			if !inConfig {
+				logger.Debug("skipping non-defaultable provider", "provider", pp.Name)
+				continue
+			}
 		}
 
 		// Skip the providers that aren't usable.
@@ -174,6 +187,7 @@ func (p *Project) DefaultProvider(opts *core.DefaultProviderOptions) (string, er
 				return "", err
 			}
 			if !usable {
+				logger.Debug("Skipping unusable provider", "provider", pp.Name)
 				continue
 			}
 		}

plugins/commands/serve/service/internal_service.rb

@@ -103,14 +103,20 @@ module VagrantPlugins
           when :PROVIDER
             _, popts = class_or_tuple_with_class_and_options
             return SDK::PluginInfo::ProviderOptions.new(
+              # Priority is always set in V2::Plugin.provider
               priority:     popts[:priority],
-              parallel:     !!popts[:parallel],
-              box_optional: !!popts[:box_optional],
-              defaultable:  !!popts[:defaultable],
+              # Parallel is passed along to Environment#batch which defaults it to true
+              parallel:     popts.fetch(:parallel, true),
+              # BoxOptional defaults to falsy when it's used in Kernel_V2::VMConfig
+              box_optional: popts.fetch(:box_optional, false),
+              # Defaultable is considered true when it is not specified in Environment#default_provider
+              defaultable:  popts.fetch(:defaultable, true),
             )
           when :PROVISIONER
+            # No options for provisioners
             return Google::Protobuf::Empty.new
           when :PUSH
+            # Push plugins accept an options hash but it's never used.
             return Google::Protobuf::Empty.new
           when :SYNCEDFOLDER
             _, sf_priority = class_or_tuple_with_class_and_options