core: don't replace insecure key on base package [GH-5310]

lib/vagrant/machine.rb

@@ -116,6 +116,9 @@ module Vagrant
       # XXX: This is temporary. This will be removed very soon.
       if base
         @id = name
+
+        # For base setups, we don't want to insert the key
+        @config.ssh.insert_key = false
       else
         reload
       end

test/unit/vagrant/machine_test.rb

@@ -70,6 +70,15 @@ describe Vagrant::Machine do
       expect(subject.id).to be_nil
     end
 
+    describe "as a base" do
+      let(:base) { true}
+
+      it "should not insert key" do
+        subject = new_instance
+        expect(subject.config.ssh.insert_key).to be_false
+      end
+    end
+
     describe "communicator loading" do
       it "doesn't eager load SSH" do
         config.vm.communicator = :ssh