ifupdown changes for active-active vxlan

Ticket: CM-3715
Reviewed By: CCR-3065
Testing Done: unit testing

Changes to proto down vxlan devices to avoid packet loops or duplicates before
clagd checks consistency and installs egress acl rules. When vxlan device is
brought up, check for presence of peer link and proto-down the device if peer
link exists.
- get_dependent_ifacenames is used to eliminate order dependency in specifying
peer link and vxlan configuration in /etc/network/interfaces.
- A separate script clagVxlanProtoDown is used to protodown vxlan devices if
peer link is configured separately (ifup peerlink) and vxlan devices are
already functional in the system.

config/addons.conf

@@ -1,7 +1,7 @@
 pre-up,ifenslave
-pre-up,clagd
 pre-up,vlan
 pre-up,vxlan
+pre-up,clagd
 pre-up,usercmds
 pre-up,bridge
 pre-up,bridgevlan