[ConfigManager] Add a new command line option: -l or --log-level.

Previously the way to set a log level is to use MLPL_LOGGER_LEVEL.
However, general user won't understanda what MLPL is.
So Haothol server gives the way to set it.
[server/test] Fixed the build error on Ubuntu 12.04.
Merge pull request #638 from project-hatohol/log-level-option

[ConfigManager] Add a new command line option: -l or --log-level.