The instance bssgp_nsi is a global instance to be used by all
NS related functions. Previous the PCU allocated and initialized
the bssgp_nsi instance when (re-)connecting and freeing on disconnect.
The problem of the implicit initialisation is gprs_ns_vty_init(bssgp_nsi).
All vty init functions must be called before the configuration is read,
otherwise a previous vty written configuration is invalid.
Furthermore the vty modifications to the `ns` object were lost when the PCU has to
reconnect to the SGSN.
Fixes: OS#4024
Change-Id: I2aa53ea54e9352577f6280ad7b9d1d9da9f57eaf