sgsn: change configuration to the new NS2
Change the whole vty configuration for NS to be more flexible
and support more setups. Old configurations are invalid.
For further information see:
https://osmocom.org/projects/libosmocore/wiki/Network_service_(NS)
Depends-on: Ie9306ab4d4738c2c57a69987086e22771b30657e (osmo-sgsn)
Change-Id: I65fb4e52d22617cf8488d8ab77c970f84995560d
