osmo-gbproxy: use 'osmo-gbproxy.cfg' as default config name

Like we did in osmo-sgsn in If804da17a7481e79e000fe40ae0d9c4be9722e61, move
from 'osmo_gbproxy.cfg' to 'osmo-gbproxy.cfg' as default config file name.
Still look for the legacy file name to not break old setups.

Change-Id: I9448908d94a23001f04b6334a78739a839b91313
1 file changed