zebra: changes to run DF election

zebra: changes to run DF election

1. DF preference is configurable per-ES
!
interface hostbond1
 evpn mh es-df-pref 100 >>>>>>>>>>>
 evpn mh es-id 1
 evpn mh es-sys-mac 00:00:00:00:01:11
!
2. This parameter is sent to BGP and advertised via the ESR.
3. The peer-ESs' DF params are sent to zebra (by BGP) and used
for running the DF election.
4. If the local VTEP becomes non-DF on an ES a block filter is
programmed in the dataplane to drop de-capsulated BUM packets
destined to that ES.

Sample output
=============
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
torm-11# sh evpn es
Type: L local, R remote, N non-DF
ESI                            Type ES-IF                 VTEPs
03:00:00:00:00:01:11:00:00:01  LRN  hostbond1             27.0.0.16
03:00:00:00:00:01:22:00:00:02  LR   hostbond2             27.0.0.16
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
torm-11# sh evpn es 03:00:00:00:00:01:11:00:00:01
ESI: 03:00:00:00:00:01:11:00:00:01
 Type: Local,Remote
 Interface: hostbond1
 State: up
 Ready for BGP: yes
 VNI Count: 10
 MAC Count: 2
 DF: status: non-df preference: 100 >>>>>>>>
 Nexthop group: 0x2000001
 VTEPs:
     27.0.0.16 df_alg: preference df_pref: 32767 nh: 0x100000d >>>>
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>