command line switch to auto-map input to output if only one output is available.

[iohzrd]

As I stated here https://github.com/Hjdskes/cage/pull/167#issuecomment-864577332, I understand wanting to avoid scenarios where things break in non-obvious ways, but perhaps a reasonable compromise could be to require a command line switch to enable this behavior.

This feature would be very useful to me because I use Cage in a production environment where I re-use a common disk image between 10+ different products (and growing), and minimizing (re)configuration saves me and my techs a lot of time.