Switch to PM2 fork mode and fix start.sh

This commit switches ems-email to use PM2's fork mode, rather than
cluster. This is required to ensure the cron_restart property works
correctly.

I also added a missing exit if an empty key is passed to start.sh

ecosystem.config.js.template

@@ -7,6 +7,7 @@ module.exports = {
             autorestart: false,
             watch: false,
             cron_restart: "*/15 * * * *",
+            exec_mode: 'fork',
             max_memory_restart: '1G',
             env: {
                 NODE_ENV: 'development',

start.sh

@@ -30,6 +30,7 @@ KEY_LEN=$(echo -n $key | wc -m)
 if [[ $KEY_LEN = 0 ]]
 then
   echo "Invalid key passed"
+  exit
 fi
 
 KEY=$key pm2 start --only $instance --env $env