core: VirtualBox is no longer the fallback, not hardcoded anywhere

This commit finally removes all traces of VirtualBox defaults from
Vagrant core. Vagrant now completely relies on its automated provider
detection to pick the best and most relevant provider that is available
on the system and for a specific development environment.

A "vagrant up" on one development environment might prefer VirtualBox,
another might prefer Docker. As a result of this commit (plus the few
prior), the developer doesn't need to know anymore and doesn't need to
specify a `--provider` flag. Vagrant just figures out the correct
provider.

Yay. :)

lib/vagrant/environment.rb

@@ -369,8 +369,8 @@ module Vagrant
         return key if impl.usable?(false)
       end
 
-      # If all else fails, return VirtualBox
-      return :virtualbox
+      # No providers available is a critical error for Vagrant.
+      raise Errors::NoDefaultProvider
     end
 
     # Returns the collection of boxes for the environment.

lib/vagrant/errors.rb

@@ -476,6 +476,10 @@ module Vagrant
       error_key(:nfs_client_not_installed_in_guest)
     end
 
+    class NoDefaultProvider < VagrantError
+      error_key(:no_default_provider)
+    end
+
     class NoDefaultSyncedFolderImpl < VagrantError
       error_key(:no_default_synced_folder_impl)
     end

templates/locales/en.yml

@@ -861,6 +861,21 @@ en:
 
         If this message is erroneous, you may disable this check by setting
         `config.nfs.verify_installed` to `false` in your Vagrantfile.
+      no_default_provider: |-
+        No usable default provider could be found for your system.
+
+        Vagrant relies on interactions with 3rd party systems, known as
+        "providers", to provide Vagrant with resources to run development
+        environments. Examples are VirtualBox, VMware, Hyper-V.
+
+        The easiest solution to this message is to install VirtualBox, which
+        is available for free on all major platforms.
+
+        If you believe you already have a provider available, make sure it
+        is properly installed and configured. You can see more details about
+        why a particular provider isn't working by forcing usage with
+        `vagrant up --provider=PROVIDER`, which should give you a more specific
+        error message for that particular provider.
       no_default_synced_folder_impl: |-
         No synced folder implementation is available for your synced folders!
         Please consult the documentation to learn why this may be the case.