bgpd: enhance network command for evpn route type 5

bgpd: enhance network command for evpn route type 5

A new vty command available under evpn address family. This command
takes following format:

(af-evpn)# [no] network <A.B.C.D/M|X:X::X:X/M> rd ASN:nn_or_IP-address:nn ethtag WORD
                 label WORD esi WORD gwip A.B.C.D routermac WORD
		 [route-map WORD]

Among new parameters, ethtag stands for the ethernet tag indentifier.
ESI stands for the ethernet segment identifier, and must be entered in
following format: 00:11:22:33:44:55:66:77:88:99.
gwip stands for the gateway IP address contained in RT5 message. A
check is done on that value since if gwip is ipv4, then ip prefix must
be ipv4. The same for ipv6.
RouterMAc is the gateway mac address sent as extended community
attribute.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>