nagios_common: Refactor custom command definitions

- Setup a check_command define
- Separate commands config into one file per set of commands
- Move command config into a folder /etc/icinga/commands
- Move check_graphite to use the new system
- Setup a class that'll house all check_commands
- Setup custom checks in shinken as well

TODO:

- Need to find a way to specify a template as well. See
  PS6 of this change for issues with that

Change-Id: I3c3bc48bf8730cd262ec32785ee6a7360f7e24aa