Add support for 'st2 apikey load'

Similar as we do with importing data into st2 keyvalue storage #30, need an option to import st2 apikeys.
Respective documentation: https://docs.stackstorm.com/authentication.html#api-key-migration

As needed in https://github.com/StackStorm/ops-infra/blob/master/roles/stackstorm.st2cicd/tasks/install_api_keys.yml