Don't check for errors when checking if a provider is usable

If a provider is unusable then Usable() will throw an error. When
checking for a default provider, Vagrant should not raise an error
if there is an issue with executing Usable().

internal/core/project.go

@@ -392,10 +392,7 @@ func (p *Project) DefaultProvider(opts *core.DefaultProviderOptions) (string, er
 		if opts.CheckUsable {
 			logger.Debug("Checking usable on provider", "provider", pp.Name)
 			pluginImpl := plug.Plugin.(core.Provider)
-			usable, err := pluginImpl.Usable()
-			if err != nil {
-				return "", err
-			}
+			usable, _ := pluginImpl.Usable()
 			if !usable {
 				logger.Debug("Skipping unusable provider", "provider", pp.Name)
 				continue

lib/vagrant/environment.rb

@@ -351,7 +351,7 @@ module Vagrant
       # then look there.
       root_config = vagrantfile.config
       if opts[:machine]
-        machine_info = vagrantfile.machine_config(opts[:machine])
+        machine_info = vagrantfile.machine_config(opts[:machine], nil, nil, nil)
         root_config = machine_info[:config]
       end