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.
メソッド名がfield名とかぶらないように `get_default_host_parameter(host)` とかしておきましょうか。
引数があり、fieldと全くおなじように扱えるわけではないので別名が良いと思います。