Move clag_enable setting to ifupdown2 ifenslave.

Ticket: CM-4802
Reviewed By:
Testing Done: yes

1. clag_enable needs to be set before slaves are added to the bond. Otherwise
bonding driver will bring the slaves up independent of clagd.
2. also added post_down clagd addon to cleanup the clag-id on ifdown.

config/addons.conf

@@ -18,6 +18,7 @@ down,dhcp
 down,addressvirtual
 down,address
 down,usercmds
+post-down,clagd
 post-down,mstpctl
 post-down,bridgevlan
 post-down,bridge