bgpd: Add '[no] flood <disable|head-end-replication>'

bgpd: Add '[no] flood <disable|head-end-replication>'

Add the '[no] flood <disable|head-end-replication>' command
to the l2vpn evpn afi/safi sub commands for bgp.  This command
when entered as 'flood disable' will turn off type 3 route
generation for the transmittal of the type 3 route necessary
for BUM replication on the remote VTEP.  Additionally it will
turn off the BUM handling via the new zebra command,
ZEBRA_VXLAN_FLOOD_CONTROL.

Signed-off-by: Vivek Venkatraman <vivek@cumulusnetworks.com>
Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>