Support NIC identifiers as alternative to MAC addresses

[GrigoriyMikhalkin]

This pull request is part of implementing support for SONiC.

Currently we are using the mac addresses on the interfaces to calculate machine connections. On SONiC switches though, all mac addresses on the switch interfaces have the same mac address. Therefore, we need some alternative identifier instead of mac addresses in order to support SONiC.

With this PR, metal-core is allowed to alternatively register its interfaces with nic identifiers, which are, if provided, used instead of mac addresses for calculating machine connections.

In order to make the identifiers for SONiC unique across racks, we additionally report Hostname with the metal-hammer. When using the identifiers, we can then narrow down nics by the switch hostnames to ensure we only have the corresponding switch nics to be used for calculating the machine connections.

The metal-hammer will:

• in the SONiC case: report nic identifier Eth14/4(Port14), no mac address
• in the Cumulus case: report nic identifier <the mac address>, the mac address as before

This is a breaking change for the metal-hammer, it has to be released along with this PR's changes.

/cc @robertvolkmann

References:

• Sonic redis with serialized interface reconfiguration metal-core#81
• Support reporting nic identifiers metal-hammer#99
• Add switch ssh|console and show more details metalctl#176

[Gerrit91]

Before merging, we should manually test this PR:

• Run in mini-lab
• Deploy to test environment
  • Ensure metal-core restart is working (switch registration)
  • Ensure older metal-core compatible with this PR
• Delete machine from database and see that it comes back properly

Done