provisioners/docker: unit tests for config

/cc @fgrehm - I'm starting to unit test more and more of the plugins.
Just showing you an example because it is quite easy. :)

test/unit/plugins/provisioners/docker/config_test.rb

@@ -0,0 +1,44 @@
+require File.expand_path("../../../../base", __FILE__)
+
+require Vagrant.source_root.join("plugins/provisioners/docker/config")
+
+describe VagrantPlugins::Docker::Config do
+  subject { described_class.new }
+
+  describe "#images" do
+    it "stores them in a set" do
+      subject.images = ["1", "1", "2"]
+      subject.finalize!
+      expect(subject.images.to_a.sort).to eql(["1", "2"])
+    end
+
+    it "overrides previously set images" do
+      subject.images = ["3"]
+      subject.images = ["1", "1", "2"]
+      subject.finalize!
+      expect(subject.images.to_a.sort).to eql(["1", "2"])
+    end
+  end
+
+  describe "#pull_images" do
+    it "adds images to the list of images to build" do
+      subject.pull_images("1")
+      subject.pull_images("2", "3")
+      subject.finalize!
+      expect(subject.images.to_a.sort).to eql(["1", "2", "3"])
+    end
+  end
+
+  describe "#version" do
+    it "defaults to latest" do
+      subject.finalize!
+      expect(subject.version).to eql(:latest)
+    end
+
+    it "converts to a symbol" do
+      subject.version = "v27"
+      subject.finalize!
+      expect(subject.version).to eql(:v27)
+    end
+  end
+end