Junos vrf support in get_arp()

[ckishimo]

Support for VRF in get_arp() following #512 in Junos

ckishimo@ex9208> show arp vpn VRF | display xml rpc    
<rpc-reply xmlns:junos="http://xml.juniper.net/junos/16.1R5/junos">
    <rpc>
        <get-arp-table-information>
                <vpn>VRF</vpn>
        </get-arp-table-information>
    </rpc>
    <cli>
        <banner>[edit]</banner>
    </cli>
</rpc-reply>

ckishimo@ex9208> show arp vpn VRF                      
MAC Address       Address         Name                      Interface               Flags
2c:21:31:a5:14:84 192.168.60.2    192.168.60.2              ge-3/1/3.0              none

[
    {
        "age": 882.0, 
        "interface": "ge-3/1/3.0", 
        "ip": "192.168.60.2", 
        "mac": "2C:21:31:A5:14:84"
    }
]

[mirceaulinic]

@ckishimo can you please rebase your branch?

[ckishimo]

@mirceaulinic oops! I just did... pls let me know if everything is in order now. Thanks

[coveralls]

Coverage decreased (-0.03%) to 78.598% when pulling 260ecdf on ckishimo:arp-vrf into 86617bc on napalm-automation:arp-vrf.