support configuring path instead of hardcoded `/var/lib/sbctl`

[thebigbone]

hello, currently it's not possible to choose a path for storing sbctl keys and config. it would be nice to, for ex, store the keys in an external mounted storage and not keep it in host all the time, even when you don't have to sign anything.

[bsingh-kpt]

@thebigbone That is default config. If you create config.json, you can point sbctl to alternate keydir.
./sbctl --config config.json

{ "keydir": "/home/user/sbkeys" }