Make role actually allow different memory configuration

WOW that took long enough

templates/env

@@ -0,0 +1,2 @@
+JRE_XMX={{ minecraft_jre_xmx }}
+JRE_XMS={{ minecraft_jre_xms }}

templates/minecraft@.service

@@ -12,6 +12,7 @@ After=network.target
 User=minecraft
 Group=minecraft
 WorkingDirectory={{ minecraft_home }}/%i
+EnvironmentFile={{ minecraft_home }}/%i/env
 PrivateUsers=true
 ProtectSystem=full
 ProtectHome=true
@@ -22,7 +23,7 @@ ProtectKernelModules=true
 # Implies MountAPIVFS=yes
 ProtectControlGroups=true
 
-ExecStart=/bin/sh -c '/usr/bin/screen -DmS %i /usr/bin/java -Xms{{ minecraft_jre_xms }} -Xmx{{ minecraft_jre_xmx }} -jar server.jar nogui'
+ExecStart=/bin/sh -c '/usr/bin/screen -DmS %i /usr/bin/java -Xms${JRE_XMS} -Xmx${JRE_XMX} -jar server.jar nogui'
 
 ExecReload=/usr/bin/screen -p 0 -S %i -X eval 'stuff "reload"\\015'