Run scripts as different system users via the API

The ploi API only supports running scripts as either ploi or root, even on machines that have additional system users.

We use system users to isolate sites and prevent sudo access from app users. When we configure an app, there are some tasks that require running commands under the system user.

Note that this is different than deployment scripts that also run under the system user. Deployment scripts run when triggered, typically when a repository has new commits. But there are a lot of other tasks that need to be run at other times, like setting up pm2 for a system user, installing global npm packages, and so forth.