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.
before_validateを呼ぶために@host.valid?を読んでいます．
@host.should be_validだと２重にチェックできてよいでしょうか？
コメントもつけておきます．
@host.should be_valid # it calls before_validate