Add check-muticast-groups.rb

This plugin checks if specific multicast groups are configured on specific interfaces. The `netstat` command is required.

The configurations can be put in the default sensu config directory and/or out of the sensu directory, as a JSON file. If the config file is not in the sensu directry, `-c PATH` option must be given.

An example config is included.

Example output:

```
% ./plugins/network/check-multicast-groups.rb -c ./plugins/network/check-multicast-groups.json
CheckMulticastGroups CRITICAL: 1 missing multicast group(s):
eth0    224.2.2.4
```