use a safer default name for virtualbox

if the same vagrantfile is up'd in the same second in the same basedir
this caused a conflict,

most notably, all jenkins jobs sit in a 'workspace' directory, which
breaks if more than one job launches from the same trigger.

plugins/providers/virtualbox/action/set_name.rb

@@ -23,7 +23,8 @@ module VagrantPlugins
           if !name
             prefix = "#{env[:root_path].basename.to_s}_#{env[:machine].name}"
             prefix.gsub!(/[^-a-z0-9_]/i, "")
-            name = prefix + "_#{Time.now.to_i}"
+            # milliseconds + random number suffix to allow for simultaneous `vagrant up` of the same box in different dirs
+            name = prefix + "_#{(Time.now.to_f * 1000.0).to_i}_#{rand(100000)}"
           end
 
           # Verify the name is not taken