Fix get_interfaces_ip for non ipv6 devices

[ogenstad]

There's a bug when running against devices which output:
% Invalid input detected at '^' marker.

When running the "show ipv6 interface command"

The current get_interfaces_ip would result in:

{
"Vlan20": {
"ipv4": {
"172.29.50.3": {
"prefix_length": 27
}
}
},
"Vlan41": {
"ipv4": {
"172.29.52.34": {
"prefix_length": 27
},
"192.168.81.34": {
"prefix_length": 24
}
}
},
"^": {
"ipv6": {}
},
"%": {
"ipv6": {}
}
}

[coveralls]

Coverage increased (+0.003%) to 78.921% when pulling 184e275 on ogenstad:ipv6_interfaces into 85610c9 on napalm-automation:develop.