Minimum number of AppServers should be a constant

[shatterednirvana]

Right now we assume that the minimum number of AppServers per node should be 1, but to support use cases where this isn't the case, we should extract it to a class constant in appscale/AppController/djinn.rb. In particular, the below code should be changed to replace all instances of 1 with the new constant.

    if time_since_last_decision > SCALEDOWN_TIME_THRESHOLD and
      !@app_info_map[app_name][:appengine].nil? and
      appservers_running > 1

      Djinn.log_debug("Removing an AppServer on this node for #{app_name}")
      remove_appserver_process(app_name)
      @last_decision[app_name] = Time.now.to_i
    elsif !@app_info_map[app_name][:appengine].nil? and
      appservers_running <= 1

      Djinn.log_debug("Only 1 AppServer is running - don't kill it")