Added methods to use configuration groups.
Added methods to use configuration groups.

* Direct access is fragile and causes issues when changing
  the data structure for parameters. Wrapped it in helper
  methods get_group_param(), list_group_params(), list_config_groups()
  and define_config_group_param().

cvubrugier/configshell-fb,Datera/configshell,agrover/configshell-fb