Make java heap size for the nodes configurable

Looks like we've been running out of heap space with the default of 512m. Given that it's suggested to use 50% of the RAM, this is now configurable.

docker-compose.base.yml

@@ -10,7 +10,7 @@ services:
       - discovery.seed_hosts=os-node-1,os-node-2
       - cluster.initial_master_nodes=os-node-1,os-node-2
       - bootstrap.memory_lock=true # along with the memlock settings below, disables swapping
-      - "OPENSEARCH_JAVA_OPTS=-Xms512m -Xmx512m" # minimum and maximum Java heap size, recommend setting both to 50% of system RAM
+      - "OPENSEARCH_JAVA_OPTS=-Xms${OPENSEARCH_HEAPSIZE:-512}m -Xmx${OPENSEARCH_HEAPSIZE:-512}m" # minimum and maximum Java heap size, recommend setting both to 50% of system RAM
       - OPENSEARCH_ADMIN_PASSWORD
       - OPENSEARCH_KIBANASERVER_PASSWORD
       - OPENSEARCH_LOGGINGUSER_PASSWORD