* Enhance grub_conf.py

- Move Grub1Config and Grub2Config to grub_conf.py
  Remove redundant functions among these 3 mappers
- Remove some useless variables and interfaces
- Update shared reducer GrubConf
  - Return empty instead of raising Exception when no GRUB being found
  - Add `get_grub_cmdlines` and return all boot cmdlines by default
- Modify the 2 associated reducers that used the old GrubConfig
  - selinux.py and user_namespaces.py
- Fix a bug in _parse_script to skip nested 'if'
- Refine some docstring
- Enhance coverage of grub_conf.py to 100%

TODO:
- Make GrubConfig not a parser but a base class for Grub1Config
  and Grub2Config
- Remove GrubConfig from related shared reducers

* Add test cases for `get_grub_cmdlines`