tests: add bgp_vpnv4_route_leak_basic

tests: add bgp_vpnv4_route_leak_basic

bgp_vrf_route_leak_basic uses "import/export vrf" commands to perform
route leaks between VRF on the r1 router. The same result can be
achieved by using the "route-target import / export" commands.

Copy bgp_vrf_route_leak_basic to bgp_vpnv4_route_leak_basic. Change
BGP configuration to handle the route leaks with "route-target import /
export". Change to retry timers.  No other changes.

Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>