Detect if a machine has both uplinks up

[majst01]

In Order to prepare the Cumulus SONiC migration we must ensure that every machine is connected physically and logically to both switches.

We could achieve this by using the lldp messages every machine sends to both switches, and the metal-core reports them back to metal-api. But actually we only calculate the Phoned Home state, regardless if one or two switches report that state.

This should be changed that:

• [ ] Phoned Home is considered as such if at least one switch is reporting as before
• [ ] Add additional infos to the MachineConnection, which connection reports LLDP messages and when
• [ ] enhance metalctl to visualize machines which have a broken uplink

[mwennrich]

To be certain metal-core could also check and report if the bgp-connection to all neighbors is up and established.