ldpd: fix issue when displaying the running configuration

ldpd: fix issue when displaying the running configuration

More than a cosmetic fix, this should prevent issues with frr-reload.py.

Before:

mpls ldp
 router-id 1.1.1.1
 !
 address-family ipv4
  discovery transport-address 1.1.1.1
  !
  interface rt1-eth0
   discovery hello holdtime 10
   discovery hello interval 3
 exit-address-family
 !
!

After:

mpls ldp
 router-id 1.1.1.1
 !
 address-family ipv4
  discovery transport-address 1.1.1.1
  !
  interface rt1-eth0
   discovery hello holdtime 10
   discovery hello interval 3
  !
 exit-address-family
 !
!

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>