CLI: support use of stagings

[Woetfin]

Is your feature request related to a problem? Please describe.
At the moment, the CLI can be used to interact with templates on the production Silverfin environment. But being able to change the host to some other than live.getsilverfin.com, should allow the interaction with a staging environment as well.

Describe the solution you'd like
Be able to change the host and interact with a staging from the CLI.

Describe alternatives you've considered
Still manual interactions on the platform needed when interacting with a staging.

Additional context
Setting a template to externally_managed on the live environment, causes this to be externally_managed on a staging as well. But at the moment you then can't interact through the CLI with these templates.