[Observium] Attention to OBS-3006 (vPC onNX-OS)

Hello,

I've done a quick patch to get vPC metrics from NX-OS devices, can you kindly take a look at it? The Jira issue is https://jira.observium.org/projects/OBS/issues/OBS-3006

For now, that adds just graphs to Status section. I can put some effort to create a full-blown vPC tab on the Device page to get even more stats for it, what do you think on that?

--- Sergei Fomin