Switch flight-job-script-api from puma to unicorn

[WilliamMcCumstie]

This PR switching job-script-api from using puma to unicorn. The key differences are:

• Unicorn can not currently reload its config because the app is preloaded, and
• Unicorn likes to control its pid_path as it has been daemonized.

To allow unicorn to control its pid_path, there are now two pid files:

• flight-service version: /o/f/v/run/service/job-script-api.pid
• Unicorn version: /o/f/v/run/job-script-api.pid

The start script reads the unicorn version of the pid and reports it back to flight-service. A similar thing happens in flight-www. This does open the possibility the two get out of sync. This issue can be revisited if and when it becomes a problem.