Allow configuring defaults per context

Right now we can use the context parameter to create different auth-contexts on the `config.yaml`, which already helps a lot, but I have a few suggestions on top of that:

1) Although stateless has it's advantages, it'd be nice to have an option to modify default behaviour to stateful.
2) It would be nice to have other settings be context aware, like `compute.ssh.ssh-key-path`, `compute.ssh.ssh-user`, `compute-ssh.ssh-port` and many other things.

The ability to have the option of setting context in a stateful way makes suggestion (2) above make even more sense. You switch the context once and all those settings are already adjusted for you, so no need to be passing them as arguments on the command-line.