Add support for NI with multiple ports

[milan-zededa]

This commit implements support for Local NI with multiple ports attached.
API changes with detailed description can be found here:
lf-edge/eve-api#53
This is a continuation to PRs:
#4098
#4113
(next PR will bring documentation updates)

In summary, network instance can be now configured with "shared" port
label, potentially matching multiple device ports. The NI routing table
will contain routes from all the selected ports.

Shared labels can be also used to restrict port-forwarding to a subset
of NI ports and to create multipath static routes (routes with multiple
possible next-hops). For every multipath route, zedrouter will use
recently added portprober to select the best port at a given time (based
on the connectivity status, cost, etc.) and also to failover to another
port when the currently used port looses connectivity.

Eden-SDN example for NI with multiple ports: lf-edge/eden#1008

[eriknordmark]

LGTM - kicking off tests in parallel with review discussion.

FWIW I see "Uplink" and "UplinkProber" in pkg/pillar/docs/zedrouter.md and in pkg/pillar/types/domainmgrtypes.go. Don't know if/when you want to change those.

[milan-zededa]

FWIW I see "Uplink" and "UplinkProber" in pkg/pillar/docs/zedrouter.md and in pkg/pillar/types/domainmgrtypes.go. Don't know if/when you want to change those.

There was an unused SwitchUplink attribute of VlanInfo in domainmgrtypes.go - I have removed it.
Documentation update will come in a separate PR. I have it already prepared in my EVE fork.