Default host parameter on simulator

Fixed #255

- old configuration
  - run.host_paramters are loaded from Host.host_parameter_definitions
- new configuration
  - run.host_parameters are loaded from Simulator.default_host_parameter(run.submitted_to).
  - In Simulator.default_host_parameter(run.submitted_to), if host_parameters are not defined, host_parameters are loaded from Host.host_parameter_definitions.

- new features
  - when Host.scheduler_type == "xsub" and Host.status is changed into :enabled, Host.host_parameter_definitions are reloaded by remote host xsub.
> saveが２回呼ばれるからtwiceなんじゃないんですか？

そのとおりです．書き直しました．
```
    it "gets host parameters by invoking 'get_host_parameters_for_xsub' when host status is changed into :enabled" do
      @host.update_attribute(:scheduler_type, "xsub")
      @host.update_attribute(:status, :disabled)
      @host.should_receive(:get_host_parameters_for_xsub)
      @host.status = :enabled
      @host.save!
    end
```