Add a function to get to the "original" environment with Vagrant

This function only works when used with the official Vagrant installer.

lib/vagrant.rb

@@ -227,6 +227,22 @@ module Vagrant
       requirements: requirements.join(", "),
       version: VERSION
   end
+
+  # This allows plugin developers to access the original environment before
+  # Vagrant even ran. This is useful when shelling out, especially to other
+  # Ruby processes.
+  #
+  # @return [Hash]
+  def self.original_env
+    {}.tap do |h|
+      ENV.each do |k,v|
+        if k.start_with?("VAGRANT_OLD_ENV")
+          key = k.sub(/^VAGRANT_OLD_ENV_/, "")
+          h[key] = v
+        end
+      end
+    end
+  end
 end
 
 # Default I18n to load the en locale

test/unit/vagrant_test.rb

@@ -98,4 +98,23 @@ describe Vagrant do
         to raise_error(Vagrant::Errors::VagrantVersionBad)
     end
   end
+
+  describe "original_env" do
+    before do
+      ENV["VAGRANT_OLD_ENV_foo"] = "test"
+      ENV["VAGRANT_OLD_ENV_bar"] = "test"
+    end
+
+    after do
+      ENV["VAGRANT_OLD_ENV_foo"] = "test"
+      ENV["VAGRANT_OLD_ENV_bar"] = "test"
+    end
+
+    it "should return the original environment" do
+      expect(Vagrant.original_env).to eq(
+        "foo" => "test",
+        "bar" => "test",
+      )
+    end
+  end
 end