[osutils/freebsd.py] code refine and unit tests working list

[iblislin]

When i wrote those code, i'm not quite understand programming in python. :(
So i will do some code refine now; comment welcomed! 😄

Working list:
Class FreeBSDUtils

• reboot
• user_exists
• create_user
  • bug [osutils/freebsd.py] param invite_group should be optional in create_user #21
• set_host_name
• set_user_password
• add_user_to_local_group
• get_user_home
• get_network_adapters
• set_static_network_config
• set_dhcp_network_config
• set_timezone
• adj_sys_time
• _add_comment
• _add_rc_conf
• chown

Also, the naming of those functions (set_ and get_) are really non-pythonic style.
Maybe we should use property.