[vagrant] Update Registry with #keys method.

This helper method to return the keys for the registry will give capability to eliminate some code. Example:

```ruby
keys = [] 
@subcommands.each { |k,v| keys << k }
keys.sort.each { |k| o.seperator "#{k}" }
```

Versus:

```ruby
@subcommands.keys.sort.each { |k| o.seperator "#{k}" }
```

lib/vagrant/registry.rb

@@ -35,6 +35,13 @@ module Vagrant
       @items.has_key?(key)
     end
     
+    # Returns an array populated with the keys of this object.
+    #
+    # @return [Array]
+    def keys
+      @items.keys
+    end
+
     # Iterate over the keyspace.
     def each(&block)
       @items.each do |key, _|