Add check-influxdb.rb

This plugin checks if an InfluxDB instance is working fine.

```
% ruby ./plugins/influxdb/check-influxdb.rb --help
Usage: ./plugins/influxdb/check-influxdb.rb (options)
    -h, --host HOST                  Host address of the InfluxDB server
    -p, --port PORT                  Port number of the InfluxDB server
    -s, --ssl                        Turn on/off SSL (default: false)
    -t, --timeout SECONDS            Seconds to wait for the connection to open or read (default: 1.0s)
% ruby ./plugins/influxdb/check-influxdb.rb
CheckInfluxDB OK: {"status"=>"ok"}
% ruby ./plugins/influxdb/check-influxdb.rb -h example.com
CheckInfluxDB CRITICAL: execution expired
```