Only allow whitelisted commands to be run

[ejholmes]

This is a spike into #966, which adds a configuration option to Empire to require commands to be whitelisted for emp run to work.

If a command is not whitelisted, emp run will have this behavior:

$ emp run migrate
error: command not whitelisted: migrate

To whitelist a command, you add it to the Procfile:

migrate: bundle exec rake db:migrate:up

It's recommended that you use the extended Procfile and set the noservice: true flag to prevent it from being scaled up (and it won't create any AWS resources).

migrate:
  command: bundle exec rake db:migrate:up
  noservice: true

In this initial spike, it allows any command in the Procfile to be run. For example, if you use the new scheduled tasks, you can spin up a one off of the scheduled task easily like so:

scheduled-job:
  command: ./bin/do-work
  cron: * * * * * *

$ emp run scheduled-job
doing work

Any extra arguments will be appended to the command provided in the procfile. For example:

migrate:
  command: ./bin/migrate
  noservice: true

exec bundle exec rake db:migrate:$1

$ emp run migrate up

TODO

• Tests

[phobologic]

👍 positional args would be awesome.