Original SR had defaults as

mme.gtp: 127.0.1.100
enb.gtp: 127.0.1.1

With this both can listen at the same port and talk to each other.

But if we set gtp addr to be the same for mme and enb, then enb listen
will fail as the listening address is already taken by mme.

Fix it with restoring explicit enb gtp loopback default.