Use eval to run the command instead of exec

`exec` is replacing the current shell to run the command, which means that
the script execution stops at that line.

Passing it to the background with `exec "$@" &` won't work either,
because the script will move directly to the next command where it
looks for the `.pid` files that have not yet been created because celery
takes a few seconds to spin up all the processes.

Using `sleep X` to remedy this seems just wrong given that

1. we can use `eval` that blocks until the command returns
2. there is no obvious benefit in sticking with `exec`

scripts/run_multi_worker_app_paas.sh

@@ -81,7 +81,7 @@ function get_celery_pids {
 }
 
 function start_application {
-  exec "$@"
+  eval "$@"
   get_celery_pids
   echo "Application process pids: ${APP_PIDS}"
 }