L2-NM: put oam-802.3ah at LAG-interface level, not member-link level

[julianL999]

It would be better to specify oam-802.3ah at the LAG-interface level, not at the member-link level. I cannot see a use-case for only configuring it on some member-links and not others within the same LAG.

[boucadair]

That was positionned as such to mimic RFC8466

           |  |        +--rw member-links
           |  |        |  +--rw member-link* [name]
           |  |        |     +--rw name                string
           |  |        |     +--rw speed?              uint32
           |  |        |     +--rw mode?               neg-mode
           |  |        |     +--rw link-mtu?           uint32
           |  |        |     +--rw oam-802.3ah-link {oam-3ah}?
           |  |        |        +--rw enabled?  boolean