Traceroute: add vrf arg

[mirceaulinic]

Solves #70

[mirceaulinic]

Weird error, that I'm not seeing in my env: https://travis-ci.org/napalm-automation/napalm-nxos/jobs/199103340

[dbarrosop]

Yeah, I have started to see that one as well. Fixing it is as simple from changing:

try:
    bla
except:
    ble

to:

try:
    bla
except Exception:
    ble

[mirceaulinic]

I have suspected that as well, but I didn't see that chunk of config catching the generic exception. I will double check.

[dbarrosop]

Here:

napalm-nxos/napalm_nxos/nxos.py

Line 306 in 8b4fe0e

except:

[mirceaulinic]

Thanks @dbarrosop

[coveralls]

Coverage remained the same at 67.219% when pulling 6505242 on solves-70 into 53721a9 on develop.