Add public_address cap for VirtualBox

Fixes GH-5978

plugins/providers/virtualbox/cap/public_address.rb

@@ -0,0 +1,15 @@
+module VagrantPlugins
+  module ProviderVirtualBox
+    module Cap
+      module PublicAddress
+        def self.public_address(machine)
+          return nil if machine.state.id != :running
+
+          ssh_info = machine.ssh_info
+          return nil if !ssh_info
+          ssh_info[:host]
+        end
+      end
+    end
+  end
+end

plugins/providers/virtualbox/plugin.rb

@@ -34,6 +34,11 @@ module VagrantPlugins
         Cap
       end
 
+      provider_capability(:virtualbox, :public_address) do
+        require_relative "cap/public_address"
+        Cap::PublicAddress
+      end
+
       provider_capability(:virtualbox, :snapshot_list) do
         require_relative "cap"
         Cap

test/unit/plugins/providers/virtualbox/cap/public_address_test.rb

@@ -0,0 +1,43 @@
+require_relative "../base"
+
+require Vagrant.source_root.join("plugins/providers/virtualbox/cap/public_address")
+
+describe VagrantPlugins::ProviderVirtualBox::Cap::PublicAddress do
+  include_context "unit"
+
+  let(:iso_env) do
+    # We have to create a Vagrantfile so there is a root path
+    env = isolated_environment
+    env.vagrantfile("")
+    env.create_vagrant_env
+  end
+
+  let(:machine) do
+    iso_env.machine(iso_env.machine_names[0], :dummy).tap do |m|
+      allow(m).to receive(:state).and_return(state)
+    end
+  end
+
+  let(:state) do
+    double(:state)
+  end
+
+  describe "#public_address" do
+    it "returns nil when the machine is not running" do
+      allow(state).to receive(:id).and_return(:not_created)
+      expect(described_class.public_address(machine)).to be(nil)
+    end
+
+    it "returns nil when there is no ssh info" do
+      allow(state).to receive(:id).and_return(:not_created)
+      allow(machine).to receive(:ssh_info).and_return(nil)
+      expect(described_class.public_address(machine)).to be(nil)
+    end
+
+    it "returns the host" do
+      allow(state).to receive(:id).and_return(:running)
+      allow(machine).to receive(:ssh_info).and_return(host: "test")
+      expect(described_class.public_address(machine)).to eq("test")
+    end
+  end
+end