Provide an option to randomize domain name

This commit lets users enable hostname randomization from their
Vagrantfile. Without this option enabled, domain creation will fail when
multiple VMs are spun up from the same Vagrantfile, due to a domain name
conflict.

spec/support/environment_helper.rb

@@ -5,6 +5,8 @@ class EnvironmentHelper
 
   attr_writer :default_prefix, :domain_name
 
+  attr_accessor :random_hostname, :name
+
   def [](value)
     self.send(value.to_sym)
   end

spec/vagrant-libvirt/action/set_name_of_domain_spec.rb

@@ -5,10 +5,16 @@ describe VagrantPlugins::ProviderLibvirt::Action::SetNameOfDomain do
     @env = EnvironmentHelper.new
   end
 
-  it "builds uniqie domain name" do
+  it "builds unique domain name" do
+    @env.random_hostname = true
     dmn = VagrantPlugins::ProviderLibvirt::Action::SetNameOfDomain.new(Object.new, @env)
     first  = dmn.build_domain_name(@env)
     second = dmn.build_domain_name(@env)
     first.should_not eq(second) 
   end
+
+  it "builds simple domain name" do
+    dmn = VagrantPlugins::ProviderLibvirt::Action::SetNameOfDomain.new(Object.new, @env)
+    dmn.build_domain_name(@env).should eq("foo_") 
+  end
 end