guests/arch: Change hostname in one command

This commit updates the procedure for changing the hostname on arch
guests to occur in a single command. Previously, setting the hostname
and adding the value of the hostname to the /etc/hosts file was done in
two different uploads. This reduces the cycle to a single upload, making
provisioning a bit faster.

Additionally, this changes the behavior of the /etc/hosts file to:

1. Not remove localhost as an alias of 127.0.0.1
2. Prepend our custom hostname before localhost

The resulting /etc/hosts file will look something like:

127.0.0.1 my-host.example.com my-host
127.0.0.1 localhost.mydomain localhost

Tested against `terrywang/archlinux` using the following Vagrantfile:

```ruby
Vagrant.configure(2) do |config|
  config.vm.box = "terrywang/archlinux"
  config.vm.hostname = "banana-ramama.example.com"

  config.vm.network "private_network", type: "dhcp"

  config.vm.network "private_network", ip: "33.33.33.10"

  config.vm.provision "file", source: "Vagrantfile", destination: "/tmp/vf"
  config.vm.provision "shell", inline: "echo hi"
end
```

plugins/guests/arch/cap/change_host_name.rb

@@ -3,12 +3,19 @@ module VagrantPlugins
     module Cap
       class ChangeHostName
         def self.change_host_name(machine, name)
-          machine.communicate.tap do |comm|
-            # Only do this if the hostname is not already set
-            if !comm.test("sudo hostname | grep '#{name}'")
-              comm.sudo("hostnamectl set-hostname #{name}")
-              comm.sudo("sed -i 's@^\\(127[.]0[.]0[.]1[[:space:]]\\+\\)@\\1#{name} @' /etc/hosts")
-            end
+          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

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

@@ -9,29 +9,30 @@ describe "VagrantPlugins::GuestArch::Cap::ChangeHostName" do
   end
 
   let(:machine) { double("machine") }
-  let(:communicator) { VagrantTests::DummyCommunicator::Communicator.new(machine) }
+  let(:comm) { VagrantTests::DummyCommunicator::Communicator.new(machine) }
 
   before do
-    allow(machine).to receive(:communicate).and_return(communicator)
+    allow(machine).to receive(:communicate).and_return(comm)
   end
 
   after do
-    communicator.verify_expectations!
+    comm.verify_expectations!
   end
 
   describe ".change_host_name" do
-    let(:hostname) { "example.com" }
+    let(:hostname) { "banana-rama.example.com" }
 
     it "sets the hostname" do
-      communicator.stub_command("sudo hostname | grep '#{hostname}'", exit_code: 1)
-      communicator.expect_command("hostnamectl set-hostname #{hostname}")
+      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
-      communicator.stub_command("sudo hostname | grep '#{hostname}'", exit_code: 0)
+      comm.stub_command("hostname | grep -w '#{hostname}'", exit_code: 0)
       described_class.change_host_name(machine, hostname)
-      expect(communicator.received_commands.size).to eq(1)
+      expect(comm.received_commands.size).to eq(1)
     end
   end
 end