Enable Puma by default instead of Unicorn

Closes #4698 (closed)

Things to note

1. With Puma, single-node zero-dowtime updates are no longer supported. Unlike Unicorn, which supported hups (and thus continued to process requests while restarting), Puma requires a full restart which definitely means downtime. The docs will be updated as part of #5091 (closed)
2. Not updating docs in this MR, but a different one. Maybe one that covers regular docs as well as HA ones as mentioned in #5091 (closed).

Development decisions log

1. Instead of doing Unicorn or Puma check at every instance, write WebServerHelper.service_name which returns the name of the service. The settings are identical, so we can use this.
2. Whenever a setting related to unicorn or puma is set or get to configure another component, use WebServerHelper.service_name
3. Don't bother using WebServerHelper.service_name for notifications because Chef won't do anything if we attempt to notify an not-enabled service.