Collect `ssh` options into an external Agent-wide `ssh` config file.

[webbnh]

We should really consider using the -F switch to ssh, which would likely allow us to avoid having to specify things like -i and the other ${ssh_opts} here. This would make the scripts' command lines simpler as well as allowing us to do things like adding options (e.g., for persistent sessions) in a single, centralized place, rather than having to go and change each individual ssh invocation.

Originally posted by @webbnh in #2566 (comment)