zebra: install flood FDB entry only if the remote VTEP asked for HER

zebra: install flood FDB entry only if the remote VTEP asked for HER

Remote VTEPs advertise the flood mode via IMET and the ingress VTEP
needs to perform head-end-replication of BUM packets to it only if the
PMSI tunnel type is set to ingress-replication. If a type-3 route is not
rxed or rxed with a mode other than ingress-replication we can skip
installation of the flood fdb entry for that L2-VNI. In that case the
remote VTEP is either not interested in BUM traffic or is using a
"static-config" based replication mode like PIM.

Sample output with HER -
=======================
root@TORS1:~# vtysh -c "show evpn vni 1000" |grep "Remote\|flood"
 Remote VTEPs for this VNI:
  27.0.0.8 flood: HER
root@TORS1:~#

Sample output with PIM-SM -
=========================
root@TORS2:~# vtysh -c "show evpn vni 1000" |grep "Remote\|flood"
 Remote VTEPs for this VNI:
  27.0.0.7 flood: -
root@TORS2:~#

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