Add juju add-cloud and update-cloud commands

[dimaqq]

Juju CLI accepts clouds as files juju add-cloud -f <file.yaml>, what would be the jubilant way of doing the same thing?

This is used in two places:

• sunbeam snap (sets up the stack)
• integration tests across heterogenous clouds

[dimaqq]

This was requested in June AMA.

[gboutry]

Can this issue also cover update-cloud?