feat(rust): add option to set gamemode for server (#3343)

lgsm/config-default/config-lgsm/rustserver/_default.cfg

@@ -16,6 +16,7 @@ appport=28082
 rconpassword="CHANGE_ME"
 rconweb="1" # Value is: 1 for the Facepunch web panel, Rustadmin desktop and Rustadmin Online; 0 for RCON tools like Rusty.
 servername="Rust"
+gamemode="vanilla" # values: vanilla, softcore ( Doc: https://wiki.facepunch.com/rust/server-gamemodes )
 maxplayers="50"
 seed="" # range: 1-2147483647, used to reproduce a procedural map.
 salt="" # range: unknown, used to recover a known setting from an existing map.
@@ -24,7 +25,7 @@ saveinterval="300" # Auto-save in seconds.
 tickrate="30" # default: 30, range: 15-100.
 
 ## Server Parameters | https://docs.linuxgsm.com/configuration/start-parameters#additional-parameters
-startparameters="-batchmode +app.listenip ${ip} +app.port ${appport} +server.ip ${ip} +server.port ${port} +server.tickrate ${tickrate} +server.hostname \"${servername}\" +server.identity \"${selfname}\" +server.seed ${seed} +server.salt ${salt} +server.maxplayers ${maxplayers} +server.worldsize ${worldsize} +server.saveinterval ${saveinterval} +rcon.web ${rconweb} +rcon.ip ${ip} +rcon.port ${rconport} +rcon.password \"${rconpassword}\" -logfile"
+startparameters="-batchmode +app.listenip ${ip} +app.port ${appport} +server.ip ${ip} +server.port ${port} +server.tickrate ${tickrate} +server.hostname \"${servername}\" +server.identity \"${selfname}\" +server.gamemode ${gamemode} +server.seed ${seed} +server.salt ${salt} +server.maxplayers ${maxplayers} +server.worldsize ${worldsize} +server.saveinterval ${saveinterval} +rcon.web ${rconweb} +rcon.ip ${ip} +rcon.port ${rconport} +rcon.password \"${rconpassword}\" -logfile"
 
 #### LinuxGSM Settings ####