Added new interface metrics: speed, mtu, duplex and isup. (#646)

[ccztux]

Example json output from an interface with the new metrics:

{
    "enp0s3": {
        "dropin": [
            0,
            "packets"
        ],
        "packets_recv": [
            178160,
            "packets"
        ],
        "bytes_recv": [
            0.2,
            "GiB"
        ],
        "duplex": "full",
        "dropout": [
            0,
            "packets"
        ],
        "bytes_sent": [
            0.02,
            "GiB"
        ],
        "errout": [
            0,
            "errors"
        ],
        "mtu": 1500,
        "packets_sent": [
            88313,
            "packets"
        ],
        "isup": true,
        "errin": [
            0,
            "errors"
        ],
        "speed": [
            1000,
            "Mbit/s"
        ]
    }
}

Example call of check_ncpa.py against the interface node:

[user@cl01]: ncpa$ ./client/check_ncpa.py -H localhost -t mytoken -M interface/enp0s3
OK: Bytes_sent was 0.02 GiB (Dropin: 0 packets, Bytes_recv: 0.20 GiB, Packets_recv: 178237 packets, Dropout: 0 packets, Errout: 0 errors, Errin: 0 errors, Packets_sent: 88404 packets, Speed: 1000 Mbit/s) | 'bytes_sent'=0.02GiB;;; 'dropin'=0;;; 'bytes_recv'=0.20GiB;;; 'packets_recv'=178237;;; 'dropout'=0;;; 'errout'=0;;; 'errin'=0;;; 'packets_sent'=88404;;; 'speed'=1000;;;