l2vpn: re-write of app network construction
l2vpn: re-write of app network construction

Yet another attempt to make this part of the code easier to read and
maintain.  It should also make it easier to move to a leader/follower
confgiruation using apps.config.

eugeneia/snabb,dpino/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,dpino/snabb,Igalia/snabbswitch,SnabbCo/snabbswitch,SnabbCo/snabbswitch,Igalia/snabb,eugeneia/snabbswitch,dpino/snabb,eugeneia/snabb,dpino/snabb,Igalia/snabb,SnabbCo/snabbswitch,eugeneia/snabb,dpino/snabbswitch,alexandergall/snabbswitch,Igalia/snabb,dpino/snabb,snabbco/snabb,eugeneia/snabb,snabbco/snabb,dpino/snabb,Igalia/snabb,SnabbCo/snabbswitch,Igalia/snabbswitch,Igalia/snabbswitch,eugeneia/snabb,snabbco/snabb,Igalia/snabb,eugeneia/snabbswitch,snabbco/snabb,Igalia/snabb,alexandergall/snabbswitch,dpino/snabbswitch,snabbco/snabb,eugeneia/snabb,alexandergall/snabbswitch,Igalia/snabbswitch,snabbco/snabb,Igalia/snabb,snabbco/snabb,alexandergall/snabbswitch,eugeneia/snabb,eugeneia/snabbswitch,dpino/snabb,alexandergall/snabbswitch,Igalia/snabb,alexandergall/snabbswitch,alexandergall/snabbswitch,dpino/snabbswitch,alexandergall/snabbswitch,eugeneia/snabbswitch,snabbco/snabb,dpino/snabb