inherit previous roles of a task for capistrano server selection.

servers.rb

# based on: http://pastie.org/293845
module Capistrano
  class Configuration
    module Servers
      alias_method :original_find_servers, :find_servers

      def find_servers(options={})
        servers = original_find_servers options

        stack_roles = task_call_frames.collect {|frame| frame.task.options[:roles]}.compact.flatten
        servers = servers.select do |server|
          if not stack_roles.empty?
            stack_roles.any? { |role| self.roles[role].include?(server) }
          else
            true
          end
        end
        servers.uniq
      end
    end
  end
end