bgpd: support for DF election in EVPN-MH

bgpd: support for DF election in EVPN-MH

DF (Designated forwarder) election is used for picking a single
BUM-traffic forwarded per-ES. RFC7432 specifies a mechanism called
service carving for DF election. However that mechanism has many
disadvantages -
1. LBs poorly.
2. Doesn't allow for a controlled failover needed in upgrade
scenarios.
3. Not easy to hw accelerate.

To fix the poor performance of service carving alternate DF mechanisms
have been proposed via the following drafts -
draft-ietf-bess-evpn-df-election-framework
draft-ietf-bess-evpn-pref-df

This commit adds support for the pref-df election mechanism which
is used as the default. Other mechanisms including service-carving
may be added later.

In this mechanism one switch on an ES is elected as DF based on the
preference value; higher preference wins with IP address acting
as the tie-breaker (lower-IP wins if pref value is the same).

Sample output
=============
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
torm-11# sh bgp l2vpn evpn es 03:00:00:00:00:01:11:00:00:01
ESI: 03:00:00:00:00:01:11:00:00:01
 Type: LR
 RD: 27.0.0.15:6
 Originator-IP: 27.0.0.15
 Local ES DF preference: 100
 VNI Count: 10
 Remote VNI Count: 10
 Inconsistent VNI VTEP Count: 0
 Inconsistencies: -
 VTEPs:
  27.0.0.16 flags: EA df_alg: preference df_pref: 32767
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
torm-11# sh bgp l2vpn evpn route esi 03:00:00:00:00:01:11:00:00:01
*> [4]:[03:00:00:00:00:01:11:00:00:01]:[32]:[27.0.0.15]
                    27.0.0.15                          32768 i
                    ET:8 ES-Import-Rt:00:00:00:00:01:11 DF: (alg: 2, pref: 100)
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

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