stellar keys generate|add should default to global config

[leighmcculloch]

The stellar keys generate|add commands currently default to the local config stored in the local directory.

I don't think it's the best default.

It's possible we should even consider un-supporting local all together.

I find that I mostly want shared keys across projects.

Networks are shared across projects already and are global by default.

Also, the defaults for which key is selected is only stored in global, so it gets buggy when changing directory outside of the project directory and the stellar keys use persists.