bgpd: store the route-distinguisher from config as a string

bgpd: store the route-distinguisher from config as a string

The route-distinguisher string can be expressed in different
ways when the AS number is part of the RD. And the configured
string value has to be kept intact.
The following vty commands store the string value internally:
- router bgp / address-family ipv4 unicast / rd vpn export <>
- router bgp / address-family l2vpn evpn / rd <>
- router bgp / address-family l2vpn evpn / vni <> / rd <>

The vty commands where RD is configured in the below places is
not considered:
- router bgp / rfapi related commands
- router bgp / address-family xxx xxx / network .. rd <>

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