MEDIUM: mcli: make the prompt mode configurable between i/p

MEDIUM: mcli: make the prompt mode configurable between i/p

Support the same syntax in master mode as in worker mode in order to
configure the prompt. The only thing is that for now the master doesn't
have a non-interactive mode and it doesn't seem necessary to implement
it, so we only support the interactive and prompt modes. However the code
was written in a way that makes it easy to change this later if desired.