fix - config/services.bitmask.yml - add env PERSIST_BRIDGE_STATE for calling obfsvpn-server with --persist true flag

For this to work, we have to merge the upstream merge request first : leap/obfsvpn!31 (merged)

this will fix the below error due to already existing keys:

Error calling ReadInConfig: Config File "obfsvpn" Not Found in "[/opt/obfsvpn]"
error while generating key:/opt/obfsvpn/data/obfs4_state.json already exists