core: clean up default logic

lib/vagrant/environment.rb

@@ -320,7 +320,7 @@ module Vagrant
       # Get the list of providers within our configuration and assign
       # a priority to each in the order they exist so that we try these first.
       config = {}
-      root_config.vm.__providers.each_with_index do |key, idx|
+      root_config.vm.__providers.reverse.each_with_index do |key, idx|
         config[key] = idx
       end
 
@@ -340,8 +340,11 @@ module Vagrant
         # Skip excluded providers
         next if opts[:exclude] && opts[:exclude].include?(key)
 
-        # Skip providers that can't be defaulted
-        next if popts.has_key?(:defaultable) && !popts[:defaultable]
+        # Skip providers that can't be defaulted, unless they're in our
+        # config, in which case someone made our decision for us.
+        if !config.has_key?(key)
+          next if popts.has_key?(:defaultable) && !popts[:defaultable]
+        end
 
         # The priority is higher if it is in our config. Otherwise, it is
         # the priority it set PLUS the length of the config to make sure it

test/unit/vagrant/environment_test.rb

@@ -810,6 +810,7 @@ VF
 
     it "is the provider in the Vagrantfile that is usable" do
       subject.vagrantfile.config.vm.provider "foo"
+      subject.vagrantfile.config.vm.provider "bar"
       subject.vagrantfile.config.vm.finalize!
 
       plugin_providers[:foo] = [provider_usable_class(true), { priority: 5 }]