[HatoholArmPluginGate] Add a new method: getPid().
[HatoholArmPluginGate] Pass the environment variable: MLPL_LOGGER_LEVEL to plugins.

Without this feature, log level may be different between Hatohol server and the
Arm Plugins.
Merge pull request #608 from project-hatohol/derive-mlpl-logger-level-to-arm-plugins

Derive mlpl logger level to arm plugins