Add Cluster class and APIs
user
```ruby
require "Storm/Base/model"
require "Storm/Base/sodserver"

module Storm
  module Storage
    class Zone < Storm::Base::Model
      attr_accessor :name

      def from_hash(h)
        super
        @name = h[:name]
      end
    end

    class Cluster < Storm::Base::Model
      attr_accessor :description
      attr_accessor :zone

      def from_hash(h)
        super
        @description = h[:description]
        @zone = Zone.new
        @zone.from_hash h[:zone]
      end

      # Get a paginated list of block storage clusters, including the zone that
      # the cluster is in.
      #
      # @param page_num [Int] page number
      # @param page_size [Int] page size
      # @return [Hash] a hash with keys: :item_count, :item_total, :page_num,
      #                :page_size, :page_total and :items (an array of
      #                Cluster objects)
      def self.list(page_num, page_size)
        data = Storm::Base::SODServer.remote_call \
                  '/Storage/Block/Cluster/list',
                  :page_num => page_num,
                  :page_size => page_size
        res = {}
        res[:item_count] = data[:item_count]
        res[:item_total] = data[:item_total]
        res[:page_num] = data[:page_num]
        res[:page_size] = data[:page_size]
        res[:page_total] = data[:page_total]
        res[:items] = data[:items].map do |i|
          cluster = Cluster.new
          cluster.from_hash i
          cluster
        end
        res
      end
    end
  end
end
```
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