zebra: changes to advertise SVI mac by default if evpn-mh is enabled

zebra: changes to advertise SVI mac by default if evpn-mh is enabled

Added support for advertising SVI MAC if EVPN-MH is enabled.

In the case of EVPN MH arp replies from an attached server can be sent to
the ES-peer. To prevent flooding of the reply the SVI MAC needs to be
advertised by default.

Note:
advertise-svi-ip could have been used as an alternate way to advertise
SVI MAC. However that config cannot be turned on if SVI IPs are
re-used (which is done to avoid wasting IP addresses in a subnet).

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