The SGSN initialises GTP with gtp_statedir of "./" which may
not be the desired path for writing the gsn_restart file.
When starting from systemd for example, we might write
to the system root.
This patch allows override via the config file.
Closes: OS#4820
Change-Id: Ib3ffb7fd6ea1d9b0286111d8c2cba9da5394ca58