improved debian support, allow commands from dict, dynamic plugin path

[MartinVerges]

Hello,

I had some trouble with your role on debian, so I updated some stuff that might help other people too.

• Debian has no lib64 directory, so I added a second one and detect the correct one for later use with nrpe_agent_plugins_path.
• nrpe_check_commands can now be added as a dict as it where documented in defaults/main.yml before but not possible
• nrpe_server_group_name allows to provide a custom name for monitoring server group
• nrpe_configuration now can be provided as a dict var

Feel free to provide me feedback or merge if it is ok for you

Best regards Martin