core: helper to get the server URL

lib/vagrant/shared_helpers.rb

@@ -8,6 +8,14 @@ module Vagrant
     !ENV["VAGRANT_NO_PLUGINS"]
   end
 
+  # Returns the URL prefix to the server.
+  #
+  # @return [String]
+  def self.server_url
+    # TODO: default
+    ENV["VAGRANT_SERVER_URL"]
+  end
+
   # The source root is the path to the root directory of the Vagrant source.
   #
   # @return [Pathname]

test/unit/vagrant/shared_helpers_test.rb

@@ -22,6 +22,14 @@ describe Vagrant do
     end
   end
 
+  describe "#server_url" do
+    it "is the VAGRANT_SERVER_URL value" do
+      with_temp_env("VAGRANT_SERVER_URL" => "foo") do
+        expect(subject.server_url).to eq("foo")
+      end
+    end
+  end
+
   describe "#user_data_path" do
     around do |example|
       env = {