Puma config that draws on the cpu assignment to determine workers

[louispt1]

This PR re-draws the puma config in ETEngine to use the cpu assignment of the container to determine workers (if WEB_CONCURRENCY is not set explicitly).

This is more of an 'example' PR for now. Before merging this PR, we should also correctly set the WEB_CONCURRENCY variable or remove it as per this issue.

If we would like to move forward with adjusting the puma configurations in our other rails applications, for the thread-safe applications (ETModel and MyETM at least) we can also allow the threads assignment in their puma configurations to be determined buy the cpu_cores assignment, matching the number of threads and workers to the number of cpus available, or explicitly setting the RAILS_MAX_THREADS variable to limit threads explicitly.

thread_count = Integer(ENV.fetch('RAILS_MAX_THREADS', cpu_cores.to_s))
threads thread_count, thread_count

This PR also introduces preload_app! to the puma config which helps save memory between workers using copy on write. This seems to make sense in any case where more than one worker might be used.

[louispt1]

This PR should not be merged. I have set up the correct configurations for the puma config for the rails applications in our environment. PRs for each change can be found here:

• Updated puma config based on number of cpus my-etm#148
• Updated puma config based on number of cpus #1600
• Updated puma config based on number of cpus etmodel#4520
• https://github.com/quintel/projects/pull/301
• Updated puma config based on number of cpus etlocal#604
• https://github.com/quintel/dbbot/pull/7

There are two categories of configuration, one for thread safe and one for non-thread safe applications.