guests/atomic: Update hostname capability

This commit does a few things:

1. Make the hostname update idempotent with `grep -w`
2. Add the given hostname to `/etc/hosts` as recommended by the docs
3. Add missing tests

plugins/guests/atomic/cap/change_host_name.rb

@@ -3,7 +3,20 @@ module VagrantPlugins
     module Cap
       class ChangeHostName
         def self.change_host_name(machine, name)
-          machine.communicate.sudo("hostnamectl set-hostname #{name}")
+          comm = machine.communicate
+
+          if !comm.test("hostname | grep -w '#{name}'", sudo: true)
+            basename = name.split(".", 2)[0]
+            comm.sudo <<-EOH
+hostnamectl set-hostname '#{name}'
+
+# Remove comments and blank lines from /etc/hosts
+sed -i'' -e 's/#.*$//' -e '/^$/d' /etc/hosts
+
+# Prepend ourselves to /etc/hosts
+sed -i'' '1i 127.0.0.1\\t#{name}\\t#{basename}' /etc/hosts
+EOH
+          end
         end
       end
     end

test/unit/plugins/guests/atomic/cap/change_host_name_test.rb

@@ -0,0 +1,38 @@
+require_relative "../../../../base"
+
+describe "VagrantPlugins::GuestAtomic::Cap::ChangeHostName" do
+  let(:described_class) do
+    VagrantPlugins::GuestAtomic::Plugin
+      .components
+      .guest_capabilities[:atomic]
+      .get(:change_host_name)
+  end
+
+  let(:machine) { double("machine") }
+  let(:comm) { VagrantTests::DummyCommunicator::Communicator.new(machine) }
+
+  before do
+    allow(machine).to receive(:communicate).and_return(comm)
+  end
+
+  after do
+    comm.verify_expectations!
+  end
+
+  describe ".change_host_name" do
+    let(:hostname) { "banana-rama.example.com" }
+
+    it "sets the hostname" do
+      comm.stub_command("hostname | grep -w '#{hostname}'", exit_code: 1)
+
+      described_class.change_host_name(machine, hostname)
+      expect(comm.received_commands[1]).to match(/hostnamectl set-hostname '#{hostname}'/)
+    end
+
+    it "does not change the hostname if already set" do
+      comm.stub_command("hostname | grep -w '#{hostname}'", exit_code: 0)
+      described_class.change_host_name(machine, hostname)
+      expect(comm.received_commands.size).to eq(1)
+    end
+  end
+end