yang: use relative path instead of absolute one for route-map

Using absolute xpath is not optimal for finding the target node. As the
route map configuration grows, the yang validation will take more time
to complete. Relative paths are much faster.

Tested with a config file that contains ~3k route map config lines.
- before:
time cat conf | vtysh
configure
log syslog errors
router bgp 65000
!
route-map RM:BGP:VPNV4:PEER:VRR9:IN10001 permit 101
 match community CL:HCC:PATH_PRIO1_EAST1
  set local-preference 15200
!
...
route-map RM:BGP:VPNV4:PEER:VRR9:IN10001 permit 1182
 match community CL:HCC:PATH_PRIO1_EAST1082
 set local-preference 16281
!

real    13m51.500s
user    0m0.522s
sys     0m4.854s

- after:
time cat conf | vtysh
...
real    0m48.390s
user    0m0.384s
sys     0m1.245s

Signed-off-by: Loïc Sang <[email protected]>